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Network Working Group J. Klensin
Request for Comments: 5321 October 2008
Obsoletes: 2821
Updates: 1123
Category: Standards Track
Simple Mail Transfer Protocol
Status of This Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Abstract
This document is a specification of the basic protocol for Internet
electronic mail transport. It consolidates, updates, and clarifies
several previous documents, making all or parts of most of them
obsolete. It covers the SMTP extension mechanisms and best practices
for the contemporary Internet, but does not provide details about
particular extensions. Although SMTP was designed as a mail
transport and delivery protocol, this specification also contains
information that is important to its use as a "mail submission"
protocol for "split-UA" (User Agent) mail reading systems and mobile
environments.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 5
1.1. Transport of Electronic Mail . . . . . . . . . . . . . . . 5
1.2. History and Context for This Document . . . . . . . . . . 5
1.3. Document Conventions . . . . . . . . . . . . . . . . . . . 6
2. The SMTP Model . . . . . . . . . . . . . . . . . . . . . . . . 7
2.1. Basic Structure . . . . . . . . . . . . . . . . . . . . . 7
2.2. The Extension Model . . . . . . . . . . . . . . . . . . . 9
2.2.1. Background . . . . . . . . . . . . . . . . . . . . . . 9
2.2.2. Definition and Registration of Extensions . . . . . . 10
2.2.3. Special Issues with Extensions . . . . . . . . . . . . 11
2.3. SMTP Terminology . . . . . . . . . . . . . . . . . . . . . 11
2.3.1. Mail Objects . . . . . . . . . . . . . . . . . . . . . 11
2.3.2. Senders and Receivers . . . . . . . . . . . . . . . . 12
2.3.3. Mail Agents and Message Stores . . . . . . . . . . . . 12
2.3.4. Host . . . . . . . . . . . . . . . . . . . . . . . . . 13
2.3.5. Domain Names . . . . . . . . . . . . . . . . . . . . . 13
2.3.6. Buffer and State Table . . . . . . . . . . . . . . . . 14
2.3.7. Commands and Replies . . . . . . . . . . . . . . . . . 14
2.3.8. Lines . . . . . . . . . . . . . . . . . . . . . . . . 14
2.3.9. Message Content and Mail Data . . . . . . . . . . . . 15
2.3.10. Originator, Delivery, Relay, and Gateway Systems . . . 15
2.3.11. Mailbox and Address . . . . . . . . . . . . . . . . . 15
2.4. General Syntax Principles and Transaction Model . . . . . 16
3. The SMTP Procedures: An Overview . . . . . . . . . . . . . . . 17
3.1. Session Initiation . . . . . . . . . . . . . . . . . . . . 18
3.2. Client Initiation . . . . . . . . . . . . . . . . . . . . 18
3.3. Mail Transactions . . . . . . . . . . . . . . . . . . . . 19
3.4. Forwarding for Address Correction or Updating . . . . . . 21
3.5. Commands for Debugging Addresses . . . . . . . . . . . . . 22
3.5.1. Overview . . . . . . . . . . . . . . . . . . . . . . . 22
3.5.2. VRFY Normal Response . . . . . . . . . . . . . . . . . 24
3.5.3. Meaning of VRFY or EXPN Success Response . . . . . . . 25
3.5.4. Semantics and Applications of EXPN . . . . . . . . . . 26
3.6. Relaying and Mail Routing . . . . . . . . . . . . . . . . 26
3.6.1. Source Routes and Relaying . . . . . . . . . . . . . . 26
3.6.2. Mail eXchange Records and Relaying . . . . . . . . . . 26
3.6.3. Message Submission Servers as Relays . . . . . . . . . 27
3.7. Mail Gatewaying . . . . . . . . . . . . . . . . . . . . . 28
3.7.1. Header Fields in Gatewaying . . . . . . . . . . . . . 28
3.7.2. Received Lines in Gatewaying . . . . . . . . . . . . . 29
3.7.3. Addresses in Gatewaying . . . . . . . . . . . . . . . 29
3.7.4. Other Header Fields in Gatewaying . . . . . . . . . . 29
3.7.5. Envelopes in Gatewaying . . . . . . . . . . . . . . . 30
3.8. Terminating Sessions and Connections . . . . . . . . . . . 30
3.9. Mailing Lists and Aliases . . . . . . . . . . . . . . . . 31
3.9.1. Alias . . . . . . . . . . . . . . . . . . . . . . . . 31
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3.9.2. List . . . . . . . . . . . . . . . . . . . . . . . . . 31
4. The SMTP Specifications . . . . . . . . . . . . . . . . . . . 32
4.1. SMTP Commands . . . . . . . . . . . . . . . . . . . . . . 32
4.1.1. Command Semantics and Syntax . . . . . . . . . . . . . 32
4.1.2. Command Argument Syntax . . . . . . . . . . . . . . . 41
4.1.3. Address Literals . . . . . . . . . . . . . . . . . . . 43
4.1.4. Order of Commands . . . . . . . . . . . . . . . . . . 44
4.1.5. Private-Use Commands . . . . . . . . . . . . . . . . . 46
4.2. SMTP Replies . . . . . . . . . . . . . . . . . . . . . . . 46
4.2.1. Reply Code Severities and Theory . . . . . . . . . . . 48
4.2.2. Reply Codes by Function Groups . . . . . . . . . . . . 50
4.2.3. Reply Codes in Numeric Order . . . . . . . . . . . . . 52
4.2.4. Reply Code 502 . . . . . . . . . . . . . . . . . . . . 53
4.2.5. Reply Codes after DATA and the Subsequent
<CRLF>.<CRLF> . . . . . . . . . . . . . . . . . . . . 53
4.3. Sequencing of Commands and Replies . . . . . . . . . . . . 54
4.3.1. Sequencing Overview . . . . . . . . . . . . . . . . . 54
4.3.2. Command-Reply Sequences . . . . . . . . . . . . . . . 55
4.4. Trace Information . . . . . . . . . . . . . . . . . . . . 57
4.5. Additional Implementation Issues . . . . . . . . . . . . . 61
4.5.1. Minimum Implementation . . . . . . . . . . . . . . . . 61
4.5.2. Transparency . . . . . . . . . . . . . . . . . . . . . 62
4.5.3. Sizes and Timeouts . . . . . . . . . . . . . . . . . . 62
4.5.3.1. Size Limits and Minimums . . . . . . . . . . . . . 62
4.5.3.1.1. Local-part . . . . . . . . . . . . . . . . . . 63
4.5.3.1.2. Domain . . . . . . . . . . . . . . . . . . . . 63
4.5.3.1.3. Path . . . . . . . . . . . . . . . . . . . . . 63
4.5.3.1.4. Command Line . . . . . . . . . . . . . . . . . 63
4.5.3.1.5. Reply Line . . . . . . . . . . . . . . . . . . 63
4.5.3.1.6. Text Line . . . . . . . . . . . . . . . . . . 63
4.5.3.1.7. Message Content . . . . . . . . . . . . . . . 63
4.5.3.1.8. Recipients Buffer . . . . . . . . . . . . . . 64
4.5.3.1.9. Treatment When Limits Exceeded . . . . . . . . 64
4.5.3.1.10. Too Many Recipients Code . . . . . . . . . . . 64
4.5.3.2. Timeouts . . . . . . . . . . . . . . . . . . . . . 65
4.5.3.2.1. Initial 220 Message: 5 Minutes . . . . . . . . 65
4.5.3.2.2. MAIL Command: 5 Minutes . . . . . . . . . . . 65
4.5.3.2.3. RCPT Command: 5 Minutes . . . . . . . . . . . 65
4.5.3.2.4. DATA Initiation: 2 Minutes . . . . . . . . . . 66
4.5.3.2.5. Data Block: 3 Minutes . . . . . . . . . . . . 66
4.5.3.2.6. DATA Termination: 10 Minutes. . . . . . . . . 66
4.5.3.2.7. Server Timeout: 5 Minutes. . . . . . . . . . . 66
4.5.4. Retry Strategies . . . . . . . . . . . . . . . . . . . 66
4.5.5. Messages with a Null Reverse-Path . . . . . . . . . . 68
5. Address Resolution and Mail Handling . . . . . . . . . . . . . 69
5.1. Locating the Target Host . . . . . . . . . . . . . . . . . 69
5.2. IPv6 and MX Records . . . . . . . . . . . . . . . . . . . 71
6. Problem Detection and Handling . . . . . . . . . . . . . . . . 71
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6.1. Reliable Delivery and Replies by Email . . . . . . . . . . 71
6.2. Unwanted, Unsolicited, and "Attack" Messages . . . . . . . 72
6.3. Loop Detection . . . . . . . . . . . . . . . . . . . . . . 73
6.4. Compensating for Irregularities . . . . . . . . . . . . . 73
7. Security Considerations . . . . . . . . . . . . . . . . . . . 75
7.1. Mail Security and Spoofing . . . . . . . . . . . . . . . . 75
7.2. "Blind" Copies . . . . . . . . . . . . . . . . . . . . . . 76
7.3. VRFY, EXPN, and Security . . . . . . . . . . . . . . . . . 76
7.4. Mail Rerouting Based on the 251 and 551 Response Codes . . 77
7.5. Information Disclosure in Announcements . . . . . . . . . 77
7.6. Information Disclosure in Trace Fields . . . . . . . . . . 78
7.7. Information Disclosure in Message Forwarding . . . . . . . 78
7.8. Resistance to Attacks . . . . . . . . . . . . . . . . . . 78
7.9. Scope of Operation of SMTP Servers . . . . . . . . . . . . 78
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 79
9. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 80
10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 81
10.1. Normative References . . . . . . . . . . . . . . . . . . . 81
10.2. Informative References . . . . . . . . . . . . . . . . . . 82
Appendix A. TCP Transport Service . . . . . . . . . . . . . . . . 85
Appendix B. Generating SMTP Commands from RFC 822 Header
Fields . . . . . . . . . . . . . . . . . . . . . . . 85
Appendix C. Source Routes . . . . . . . . . . . . . . . . . . . . 86
Appendix D. Scenarios . . . . . . . . . . . . . . . . . . . . . . 87
D.1. A Typical SMTP Transaction Scenario . . . . . . . . . . . 88
D.2. Aborted SMTP Transaction Scenario . . . . . . . . . . . . 89
D.3. Relayed Mail Scenario . . . . . . . . . . . . . . . . . . 90
D.4. Verifying and Sending Scenario . . . . . . . . . . . . . . 92
Appendix E. Other Gateway Issues . . . . . . . . . . . . . . . . 92
Appendix F. Deprecated Features of RFC 821 . . . . . . . . . . . 93
F.1. TURN . . . . . . . . . . . . . . . . . . . . . . . . . . . 93
F.2. Source Routing . . . . . . . . . . . . . . . . . . . . . . 93
F.3. HELO . . . . . . . . . . . . . . . . . . . . . . . . . . . 93
F.4. #-literals . . . . . . . . . . . . . . . . . . . . . . . . 94
F.5. Dates and Years . . . . . . . . . . . . . . . . . . . . . 94
F.6. Sending versus Mailing . . . . . . . . . . . . . . . . . . 94
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1. Introduction
1.1. Transport of Electronic Mail
The objective of the Simple Mail Transfer Protocol (SMTP) is to
transfer mail reliably and efficiently.
SMTP is independent of the particular transmission subsystem and
requires only a reliable ordered data stream channel. While this
document specifically discusses transport over TCP, other transports
are possible. Appendices to RFC 821 [1] describe some of them.
An important feature of SMTP is its capability to transport mail
across multiple networks, usually referred to as "SMTP mail relaying"
(see Section 3.6). A network consists of the mutually-TCP-accessible
hosts on the public Internet, the mutually-TCP-accessible hosts on a
firewall-isolated TCP/IP Intranet, or hosts in some other LAN or WAN
environment utilizing a non-TCP transport-level protocol. Using
SMTP, a process can transfer mail to another process on the same
network or to some other network via a relay or gateway process
accessible to both networks.
In this way, a mail message may pass through a number of intermediate
relay or gateway hosts on its path from sender to ultimate recipient.
The Mail eXchanger mechanisms of the domain name system (RFC 1035
[2], RFC 974 [12], and Section 5 of this document) are used to
identify the appropriate next-hop destination for a message being
transported.
1.2. History and Context for This Document
This document is a specification of the basic protocol for the
Internet electronic mail transport. It consolidates, updates and
clarifies, but does not add new or change existing functionality of
the following:
o the original SMTP (Simple Mail Transfer Protocol) specification of
RFC 821 [1],
o domain name system requirements and implications for mail
transport from RFC 1035 [2] and RFC 974 [12],
o the clarifications and applicability statements in RFC 1123 [3],
and
o material drawn from the SMTP Extension mechanisms in RFC 1869
[13].
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o Editorial and clarification changes to RFC 2821 [14] to bring that
specification to Draft Standard.
It obsoletes RFC 821, RFC 974, RFC 1869, and RFC 2821 and updates RFC
1123 (replacing the mail transport materials of RFC 1123). However,
RFC 821 specifies some features that were not in significant use in
the Internet by the mid-1990s and (in appendices) some additional
transport models. Those sections are omitted here in the interest of
clarity and brevity; readers needing them should refer to RFC 821.
It also includes some additional material from RFC 1123 that required
amplification. This material has been identified in multiple ways,
mostly by tracking flaming on various lists and newsgroups and
problems of unusual readings or interpretations that have appeared as
the SMTP extensions have been deployed. Where this specification
moves beyond consolidation and actually differs from earlier
documents, it supersedes them technically as well as textually.
Although SMTP was designed as a mail transport and delivery protocol,
this specification also contains information that is important to its
use as a "mail submission" protocol, as recommended for Post Office
Protocol (POP) (RFC 937 [15], RFC 1939 [16]) and IMAP (RFC 3501
[17]). In general, the separate mail submission protocol specified
in RFC 4409 [18] is now preferred to direct use of SMTP; more
discussion of that subject appears in that document.
Section 2.3 provides definitions of terms specific to this document.
Except when the historical terminology is necessary for clarity, this
document uses the current 'client' and 'server' terminology to
identify the sending and receiving SMTP processes, respectively.
A companion document, RFC 5322 [4], discusses message header sections
and bodies and specifies formats and structures for them.
1.3. Document Conventions
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [5]. As each
of these terms was intentionally and carefully chosen to improve the
interoperability of email, each use of these terms is to be treated
as a conformance requirement.
Because this document has a long history and to avoid the risk of
various errors and of confusing readers and documents that point to
this one, most examples and the domain names they contain are
preserved from RFC 2821. Readers are cautioned that these are
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illustrative examples that should not actually be used in either code
or configuration files.
2. The SMTP Model
2.1. Basic Structure
The SMTP design can be pictured as:
+----------+ +----------+
+------+ | | | |
| User |<-->| | SMTP | |
+------+ | Client- |Commands/Replies| Server- |
+------+ | SMTP |<-------------->| SMTP | +------+
| File |<-->| | and Mail | |<-->| File |
|System| | | | | |System|
+------+ +----------+ +----------+ +------+
SMTP client SMTP server
When an SMTP client has a message to transmit, it establishes a two-
way transmission channel to an SMTP server. The responsibility of an
SMTP client is to transfer mail messages to one or more SMTP servers,
or report its failure to do so.
The means by which a mail message is presented to an SMTP client, and
how that client determines the identifier(s) ("names") of the
domain(s) to which mail messages are to be transferred, is a local
matter, and is not addressed by this document. In some cases, the
designated domain(s), or those determined by an SMTP client, will
identify the final destination(s) of the mail message. In other
cases, common with SMTP clients associated with implementations of
the POP (RFC 937 [15], RFC 1939 [16]) or IMAP (RFC 3501 [17])
protocols, or when the SMTP client is inside an isolated transport
service environment, the domain determined will identify an
intermediate destination through which all mail messages are to be
relayed. SMTP clients that transfer all traffic regardless of the
target domains associated with the individual messages, or that do
not maintain queues for retrying message transmissions that initially
cannot be completed, may otherwise conform to this specification but
are not considered fully-capable. Fully-capable SMTP
implementations, including the relays used by these less capable
ones, and their destinations, are expected to support all of the
queuing, retrying, and alternate address functions discussed in this
specification. In many situations and configurations, the less-
capable clients discussed above SHOULD be using the message
submission protocol (RFC 4409 [18]) rather than SMTP.
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The means by which an SMTP client, once it has determined a target
domain, determines the identity of an SMTP server to which a copy of
a message is to be transferred, and then performs that transfer, is
covered by this document. To effect a mail transfer to an SMTP
server, an SMTP client establishes a two-way transmission channel to
that SMTP server. An SMTP client determines the address of an
appropriate host running an SMTP server by resolving a destination
domain name to either an intermediate Mail eXchanger host or a final
target host.
An SMTP server may be either the ultimate destination or an
intermediate "relay" (that is, it may assume the role of an SMTP
client after receiving the message) or "gateway" (that is, it may
transport the message further using some protocol other than SMTP).
SMTP commands are generated by the SMTP client and sent to the SMTP
server. SMTP replies are sent from the SMTP server to the SMTP
client in response to the commands.
In other words, message transfer can occur in a single connection
between the original SMTP-sender and the final SMTP-recipient, or can
occur in a series of hops through intermediary systems. In either
case, once the server has issued a success response at the end of the
mail data, a formal handoff of responsibility for the message occurs:
the protocol requires that a server MUST accept responsibility for
either delivering the message or properly reporting the failure to do
so (see Sections 6.1, 6.2, and 7.8, below).
Once the transmission channel is established and initial handshaking
is completed, the SMTP client normally initiates a mail transaction.
Such a transaction consists of a series of commands to specify the
originator and destination of the mail and transmission of the
message content (including any lines in the header section or other
structure) itself. When the same message is sent to multiple
recipients, this protocol encourages the transmission of only one
copy of the data for all recipients at the same destination (or
intermediate relay) host.
The server responds to each command with a reply; replies may
indicate that the command was accepted, that additional commands are
expected, or that a temporary or permanent error condition exists.
Commands specifying the sender or recipients may include server-
permitted SMTP service extension requests, as discussed in
Section 2.2. The dialog is purposely lock-step, one-at-a-time,
although this can be modified by mutually agreed upon extension
requests such as command pipelining (RFC 2920 [19]).
Once a given mail message has been transmitted, the client may either
request that the connection be shut down or may initiate other mail
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transactions. In addition, an SMTP client may use a connection to an
SMTP server for ancillary services such as verification of email
addresses or retrieval of mailing list subscriber addresses.
As suggested above, this protocol provides mechanisms for the
transmission of mail. Historically, this transmission normally
occurred directly from the sending user's host to the receiving
user's host when the two hosts are connected to the same transport
service. When they are not connected to the same transport service,
transmission occurs via one or more relay SMTP servers. A very
common case in the Internet today involves submission of the original
message to an intermediate, "message submission" server, which is
similar to a relay but has some additional properties; such servers
are discussed in Section 2.3.10 and at some length in RFC 4409 [18].
An intermediate host that acts as either an SMTP relay or as a
gateway into some other transmission environment is usually selected
through the use of the domain name service (DNS) Mail eXchanger
mechanism.
Usually, intermediate hosts are determined via the DNS MX record, not
by explicit "source" routing (see Section 5 and Appendix C and
Appendix F.2).
2.2. The Extension Model
2.2.1. Background
In an effort that started in 1990, approximately a decade after RFC
821 was completed, the protocol was modified with a "service
extensions" model that permits the client and server to agree to
utilize shared functionality beyond the original SMTP requirements.
The SMTP extension mechanism defines a means whereby an extended SMTP
client and server may recognize each other, and the server can inform
the client as to the service extensions that it supports.
Contemporary SMTP implementations MUST support the basic extension
mechanisms. For instance, servers MUST support the EHLO command even
if they do not implement any specific extensions and clients SHOULD
preferentially utilize EHLO rather than HELO. (However, for
compatibility with older conforming implementations, SMTP clients and
servers MUST support the original HELO mechanisms as a fallback.)
Unless the different characteristics of HELO must be identified for
interoperability purposes, this document discusses only EHLO.
SMTP is widely deployed and high-quality implementations have proven
to be very robust. However, the Internet community now considers
some services to be important that were not anticipated when the
protocol was first designed. If support for those services is to be
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added, it must be done in a way that permits older implementations to
continue working acceptably. The extension framework consists of:
o The SMTP command EHLO, superseding the earlier HELO,
o a registry of SMTP service extensions,
o additional parameters to the SMTP MAIL and RCPT commands, and
o optional replacements for commands defined in this protocol, such
as for DATA in non-ASCII transmissions (RFC 3030 [20]).
SMTP's strength comes primarily from its simplicity. Experience with
many protocols has shown that protocols with few options tend towards
ubiquity, whereas protocols with many options tend towards obscurity.
Each and every extension, regardless of its benefits, must be
carefully scrutinized with respect to its implementation, deployment,
and interoperability costs. In many cases, the cost of extending the
SMTP service will likely outweigh the benefit.
2.2.2. Definition and Registration of Extensions
The IANA maintains a registry of SMTP service extensions. A
corresponding EHLO keyword value is associated with each extension.
Each service extension registered with the IANA must be defined in a
formal Standards-Track or IESG-approved Experimental protocol
document. The definition must include:
o the textual name of the SMTP service extension;
o the EHLO keyword value associated with the extension;
o the syntax and possible values of parameters associated with the
EHLO keyword value;
o any additional SMTP verbs associated with the extension
(additional verbs will usually be, but are not required to be, the
same as the EHLO keyword value);
o any new parameters the extension associates with the MAIL or RCPT
verbs;
o a description of how support for the extension affects the
behavior of a server and client SMTP; and
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o the increment by which the extension is increasing the maximum
length of the commands MAIL and/or RCPT, over that specified in
this Standard.
In addition, any EHLO keyword value starting with an upper or lower
case "X" refers to a local SMTP service extension used exclusively
through bilateral agreement. Keywords beginning with "X" MUST NOT be
used in a registered service extension. Conversely, keyword values
presented in the EHLO response that do not begin with "X" MUST
correspond to a Standard, Standards-Track, or IESG-approved
Experimental SMTP service extension registered with IANA. A
conforming server MUST NOT offer non-"X"-prefixed keyword values that
are not described in a registered extension.
Additional verbs and parameter names are bound by the same rules as
EHLO keywords; specifically, verbs beginning with "X" are local
extensions that may not be registered or standardized. Conversely,
verbs not beginning with "X" must always be registered.
2.2.3. Special Issues with Extensions
Extensions that change fairly basic properties of SMTP operation are
permitted. The text in other sections of this document must be
understood in that context. In particular, extensions can change the
minimum limits specified in Section 4.5.3, can change the ASCII
character set requirement as mentioned above, or can introduce some
optional modes of message handling.
In particular, if an extension implies that the delivery path
normally supports special features of that extension, and an
intermediate SMTP system finds a next hop that does not support the
required extension, it MAY choose, based on the specific extension
and circumstances, to requeue the message and try later and/or try an
alternate MX host. If this strategy is employed, the timeout to fall
back to an unextended format (if one is available) SHOULD be less
than the normal timeout for bouncing as undeliverable (e.g., if
normal timeout is three days, the requeue timeout before attempting
to transmit the mail without the extension might be one day).
2.3. SMTP Terminology
2.3.1. Mail Objects
SMTP transports a mail object. A mail object contains an envelope
and content.
The SMTP envelope is sent as a series of SMTP protocol units
(described in Section 3). It consists of an originator address (to
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which error reports should be directed), one or more recipient
addresses, and optional protocol extension material. Historically,
variations on the reverse-path (originator) address specification
command (MAIL) could be used to specify alternate delivery modes,
such as immediate display; those variations have now been deprecated
(see Appendix F and Appendix F.6).
The SMTP content is sent in the SMTP DATA protocol unit and has two
parts: the header section and the body. If the content conforms to
other contemporary standards, the header section consists of a
collection of header fields, each consisting of a header name, a
colon, and data, structured as in the message format specification
(RFC 5322 [4]); the body, if structured, is defined according to MIME
(RFC 2045 [21]). The content is textual in nature, expressed using
the US-ASCII repertoire [6]. Although SMTP extensions (such as
"8BITMIME", RFC 1652 [22]) may relax this restriction for the content
body, the content header fields are always encoded using the US-ASCII
repertoire. Two MIME extensions (RFC 2047 [23] and RFC 2231 [24])
define an algorithm for representing header values outside the US-
ASCII repertoire, while still encoding them using the US-ASCII
repertoire.
2.3.2. Senders and Receivers
In RFC 821, the two hosts participating in an SMTP transaction were
described as the "SMTP-sender" and "SMTP-receiver". This document
has been changed to reflect current industry terminology and hence
refers to them as the "SMTP client" (or sometimes just "the client")
and "SMTP server" (or just "the server"), respectively. Since a
given host may act both as server and client in a relay situation,
"receiver" and "sender" terminology is still used where needed for
clarity.
2.3.3. Mail Agents and Message Stores
Additional mail system terminology became common after RFC 821 was
published and, where convenient, is used in this specification. In
particular, SMTP servers and clients provide a mail transport service
and therefore act as "Mail Transfer Agents" (MTAs). "Mail User
Agents" (MUAs or UAs) are normally thought of as the sources and
targets of mail. At the source, an MUA might collect mail to be
transmitted from a user and hand it off to an MTA; the final
("delivery") MTA would be thought of as handing the mail off to an
MUA (or at least transferring responsibility to it, e.g., by
depositing the message in a "message store"). However, while these
terms are used with at least the appearance of great precision in
other environments, the implied boundaries between MUAs and MTAs
often do not accurately match common, and conforming, practices with
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Internet mail. Hence, the reader should be cautious about inferring
the strong relationships and responsibilities that might be implied
if these terms were used elsewhere.
2.3.4. Host
For the purposes of this specification, a host is a computer system
attached to the Internet (or, in some cases, to a private TCP/IP
network) and supporting the SMTP protocol. Hosts are known by names
(see the next section); they SHOULD NOT be identified by numerical
addresses, i.e., by address literals as described in Section 4.1.2.
2.3.5. Domain Names
A domain name (or often just a "domain") consists of one or more
components, separated by dots if more than one appears. In the case
of a top-level domain used by itself in an email address, a single
string is used without any dots. This makes the requirement,
described in more detail below, that only fully-qualified domain
names appear in SMTP transactions on the public Internet,
particularly important where top-level domains are involved. These
components ("labels" in DNS terminology, RFC 1035 [2]) are restricted
for SMTP purposes to consist of a sequence of letters, digits, and
hyphens drawn from the ASCII character set [6]. Domain names are
used as names of hosts and of other entities in the domain name
hierarchy. For example, a domain may refer to an alias (label of a
CNAME RR) or the label of Mail eXchanger records to be used to
deliver mail instead of representing a host name. See RFC 1035 [2]
and Section 5 of this specification.
The domain name, as described in this document and in RFC 1035 [2],
is the entire, fully-qualified name (often referred to as an "FQDN").
A domain name that is not in FQDN form is no more than a local alias.
Local aliases MUST NOT appear in any SMTP transaction.
Only resolvable, fully-qualified domain names (FQDNs) are permitted
when domain names are used in SMTP. In other words, names that can
be resolved to MX RRs or address (i.e., A or AAAA) RRs (as discussed
in Section 5) are permitted, as are CNAME RRs whose targets can be
resolved, in turn, to MX or address RRs. Local nicknames or
unqualified names MUST NOT be used. There are two exceptions to the
rule requiring FQDNs:
o The domain name given in the EHLO command MUST be either a primary
host name (a domain name that resolves to an address RR) or, if
the host has no name, an address literal, as described in
Section 4.1.3 and discussed further in the EHLO discussion of
Section 4.1.4.
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o The reserved mailbox name "postmaster" may be used in a RCPT
command without domain qualification (see Section 4.1.1.3) and
MUST be accepted if so used.
2.3.6. Buffer and State Table
SMTP sessions are stateful, with both parties carefully maintaining a
common view of the current state. In this document, we model this
state by a virtual "buffer" and a "state table" on the server that
may be used by the client to, for example, "clear the buffer" or
"reset the state table", causing the information in the buffer to be
discarded and the state to be returned to some previous state.
2.3.7. Commands and Replies
SMTP commands and, unless altered by a service extension, message
data, are transmitted from the sender to the receiver via the
transmission channel in "lines".
An SMTP reply is an acknowledgment (positive or negative) sent in
"lines" from receiver to sender via the transmission channel in
response to a command. The general form of a reply is a numeric
completion code (indicating failure or success) usually followed by a
text string. The codes are for use by programs and the text is
usually intended for human users. RFC 3463 [25], specifies further
structuring of the reply strings, including the use of supplemental
and more specific completion codes (see also RFC 5248 [26]).
2.3.8. Lines
Lines consist of zero or more data characters terminated by the
sequence ASCII character "CR" (hex value 0D) followed immediately by
ASCII character "LF" (hex value 0A). This termination sequence is
denoted as <CRLF> in this document. Conforming implementations MUST
NOT recognize or generate any other character or character sequence
as a line terminator. Limits MAY be imposed on line lengths by
servers (see Section 4).
In addition, the appearance of "bare" "CR" or "LF" characters in text
(i.e., either without the other) has a long history of causing
problems in mail implementations and applications that use the mail
system as a tool. SMTP client implementations MUST NOT transmit
these characters except when they are intended as line terminators
and then MUST, as indicated above, transmit them only as a <CRLF>
sequence.
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2.3.9. Message Content and Mail Data
The terms "message content" and "mail data" are used interchangeably
in this document to describe the material transmitted after the DATA
command is accepted and before the end of data indication is
transmitted. Message content includes the message header section and
the possibly structured message body. The MIME specification (RFC
2045 [21]) provides the standard mechanisms for structured message
bodies.
2.3.10. Originator, Delivery, Relay, and Gateway Systems
This specification makes a distinction among four types of SMTP
systems, based on the role those systems play in transmitting
electronic mail. An "originating" system (sometimes called an SMTP
originator) introduces mail into the Internet or, more generally,
into a transport service environment. A "delivery" SMTP system is
one that receives mail from a transport service environment and
passes it to a mail user agent or deposits it in a message store that
a mail user agent is expected to subsequently access. A "relay" SMTP
system (usually referred to just as a "relay") receives mail from an
SMTP client and transmits it, without modification to the message
data other than adding trace information, to another SMTP server for
further relaying or for delivery.
A "gateway" SMTP system (usually referred to just as a "gateway")
receives mail from a client system in one transport environment and
transmits it to a server system in another transport environment.
Differences in protocols or message semantics between the transport
environments on either side of a gateway may require that the gateway
system perform transformations to the message that are not permitted
to SMTP relay systems. For the purposes of this specification,
firewalls that rewrite addresses should be considered as gateways,
even if SMTP is used on both sides of them (see RFC 2979 [27]).
2.3.11. Mailbox and Address
As used in this specification, an "address" is a character string
that identifies a user to whom mail will be sent or a location into
which mail will be deposited. The term "mailbox" refers to that
depository. The two terms are typically used interchangeably unless
the distinction between the location in which mail is placed (the
mailbox) and a reference to it (the address) is important. An
address normally consists of user and domain specifications. The
standard mailbox naming convention is defined to be
"local-part@domain"; contemporary usage permits a much broader set of
applications than simple "user names". Consequently, and due to a
long history of problems when intermediate hosts have attempted to
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optimize transport by modifying them, the local-part MUST be
interpreted and assigned semantics only by the host specified in the
domain part of the address.
2.4. General Syntax Principles and Transaction Model
SMTP commands and replies have a rigid syntax. All commands begin
with a command verb. All replies begin with a three digit numeric
code. In some commands and replies, arguments are required following
the verb or reply code. Some commands do not accept arguments (after
the verb), and some reply codes are followed, sometimes optionally,
by free form text. In both cases, where text appears, it is
separated from the verb or reply code by a space character. Complete
definitions of commands and replies appear in Section 4.
Verbs and argument values (e.g., "TO:" or "to:" in the RCPT command
and extension name keywords) are not case sensitive, with the sole
exception in this specification of a mailbox local-part (SMTP
Extensions may explicitly specify case-sensitive elements). That is,
a command verb, an argument value other than a mailbox local-part,
and free form text MAY be encoded in upper case, lower case, or any
mixture of upper and lower case with no impact on its meaning. The
local-part of a mailbox MUST BE treated as case sensitive.
Therefore, SMTP implementations MUST take care to preserve the case
of mailbox local-parts. In particular, for some hosts, the user
"smith" is different from the user "Smith". However, exploiting the
case sensitivity of mailbox local-parts impedes interoperability and
is discouraged. Mailbox domains follow normal DNS rules and are
hence not case sensitive.
A few SMTP servers, in violation of this specification (and RFC 821)
require that command verbs be encoded by clients in upper case.
Implementations MAY wish to employ this encoding to accommodate those
servers.
The argument clause consists of a variable-length character string
ending with the end of the line, i.e., with the character sequence
<CRLF>. The receiver will take no action until this sequence is
received.
The syntax for each command is shown with the discussion of that
command. Common elements and parameters are shown in Section 4.1.2.
Commands and replies are composed of characters from the ASCII
character set [6]. When the transport service provides an 8-bit byte
(octet) transmission channel, each 7-bit character is transmitted,
right justified, in an octet with the high-order bit cleared to zero.
More specifically, the unextended SMTP service provides 7-bit
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transport only. An originating SMTP client that has not successfully
negotiated an appropriate extension with a particular server (see the
next paragraph) MUST NOT transmit messages with information in the
high-order bit of octets. If such messages are transmitted in
violation of this rule, receiving SMTP servers MAY clear the high-
order bit or reject the message as invalid. In general, a relay SMTP
SHOULD assume that the message content it has received is valid and,
assuming that the envelope permits doing so, relay it without
inspecting that content. Of course, if the content is mislabeled and
the data path cannot accept the actual content, this may result in
the ultimate delivery of a severely garbled message to the recipient.
Delivery SMTP systems MAY reject such messages, or return them as
undeliverable, rather than deliver them. In the absence of a server-
offered extension explicitly permitting it, a sending SMTP system is
not permitted to send envelope commands in any character set other
than US-ASCII. Receiving systems SHOULD reject such commands,
normally using "500 syntax error - invalid character" replies.
8-bit message content transmission MAY be requested of the server by
a client using extended SMTP facilities, notably the "8BITMIME"
extension, RFC 1652 [22]. 8BITMIME SHOULD be supported by SMTP
servers. However, it MUST NOT be construed as authorization to
transmit unrestricted 8-bit material, nor does 8BITMIME authorize
transmission of any envelope material in other than ASCII. 8BITMIME
MUST NOT be requested by senders for material with the high bit on
that is not in MIME format with an appropriate content-transfer
encoding; servers MAY reject such messages.
The metalinguistic notation used in this document corresponds to the
"Augmented BNF" used in other Internet mail system documents. The
reader who is not familiar with that syntax should consult the ABNF
specification in RFC 5234 [7]. Metalanguage terms used in running
text are surrounded by pointed brackets (e.g., <CRLF>) for clarity.
The reader is cautioned that the grammar expressed in the
metalanguage is not comprehensive. There are many instances in which
provisions in the text constrain or otherwise modify the syntax or
semantics implied by the grammar.
3. The SMTP Procedures: An Overview
This section contains descriptions of the procedures used in SMTP:
session initiation, mail transaction, forwarding mail, verifying
mailbox names and expanding mailing lists, and opening and closing
exchanges. Comments on relaying, a note on mail domains, and a
discussion of changing roles are included at the end of this section.
Several complete scenarios are presented in Appendix D.
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3.1. Session Initiation
An SMTP session is initiated when a client opens a connection to a
server and the server responds with an opening message.
SMTP server implementations MAY include identification of their
software and version information in the connection greeting reply
after the 220 code, a practice that permits more efficient isolation
and repair of any problems. Implementations MAY make provision for
SMTP servers to disable the software and version announcement where
it causes security concerns. While some systems also identify their
contact point for mail problems, this is not a substitute for
maintaining the required "postmaster" address (see Section 4).
The SMTP protocol allows a server to formally reject a mail session
while still allowing the initial connection as follows: a 554
response MAY be given in the initial connection opening message
instead of the 220. A server taking this approach MUST still wait
for the client to send a QUIT (see Section 4.1.1.10) before closing
the connection and SHOULD respond to any intervening commands with
"503 bad sequence of commands". Since an attempt to make an SMTP
connection to such a system is probably in error, a server returning
a 554 response on connection opening SHOULD provide enough
information in the reply text to facilitate debugging of the sending
system.
3.2. Client Initiation
Once the server has sent the greeting (welcoming) message and the
client has received it, the client normally sends the EHLO command to
the server, indicating the client's identity. In addition to opening
the session, use of EHLO indicates that the client is able to process
service extensions and requests that the server provide a list of the
extensions it supports. Older SMTP systems that are unable to
support service extensions, and contemporary clients that do not
require service extensions in the mail session being initiated, MAY
use HELO instead of EHLO. Servers MUST NOT return the extended EHLO-
style response to a HELO command. For a particular connection
attempt, if the server returns a "command not recognized" response to
EHLO, the client SHOULD be able to fall back and send HELO.
In the EHLO command, the host sending the command identifies itself;
the command may be interpreted as saying "Hello, I am <domain>" (and,
in the case of EHLO, "and I support service extension requests").
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3.3. Mail Transactions
There are three steps to SMTP mail transactions. The transaction
starts with a MAIL command that gives the sender identification. (In
general, the MAIL command may be sent only when no mail transaction
is in progress; see Section 4.1.4.) A series of one or more RCPT
commands follows, giving the receiver information. Then, a DATA
command initiates transfer of the mail data and is terminated by the
"end of mail" data indicator, which also confirms the transaction.
The first step in the procedure is the MAIL command.
MAIL FROM:<reverse-path> [SP <mail-parameters> ] <CRLF>
This command tells the SMTP-receiver that a new mail transaction is
starting and to reset all its state tables and buffers, including any
recipients or mail data. The <reverse-path> portion of the first or
only argument contains the source mailbox (between "<" and ">"
brackets), which can be used to report errors (see Section 4.2 for a
discussion of error reporting). If accepted, the SMTP server returns
a "250 OK" reply. If the mailbox specification is not acceptable for
some reason, the server MUST return a reply indicating whether the
failure is permanent (i.e., will occur again if the client tries to
send the same address again) or temporary (i.e., the address might be
accepted if the client tries again later). Despite the apparent
scope of this requirement, there are circumstances in which the
acceptability of the reverse-path may not be determined until one or
more forward-paths (in RCPT commands) can be examined. In those
cases, the server MAY reasonably accept the reverse-path (with a 250
reply) and then report problems after the forward-paths are received
and examined. Normally, failures produce 550 or 553 replies.
Historically, the <reverse-path> was permitted to contain more than
just a mailbox; however, contemporary systems SHOULD NOT use source
routing (see Appendix C).
The optional <mail-parameters> are associated with negotiated SMTP
service extensions (see Section 2.2).
The second step in the procedure is the RCPT command. This step of
the procedure can be repeated any number of times.
RCPT TO:<forward-path> [ SP <rcpt-parameters> ] <CRLF>
The first or only argument to this command includes a forward-path
(normally a mailbox and domain, always surrounded by "<" and ">"
brackets) identifying one recipient. If accepted, the SMTP server
returns a "250 OK" reply and stores the forward-path. If the
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recipient is known not to be a deliverable address, the SMTP server
returns a 550 reply, typically with a string such as "no such user -
" and the mailbox name (other circumstances and reply codes are
possible).
The <forward-path> can contain more than just a mailbox.
Historically, the <forward-path> was permitted to contain a source
routing list of hosts and the destination mailbox; however,
contemporary SMTP clients SHOULD NOT utilize source routes (see
Appendix C). Servers MUST be prepared to encounter a list of source
routes in the forward-path, but they SHOULD ignore the routes or MAY
decline to support the relaying they imply. Similarly, servers MAY
decline to accept mail that is destined for other hosts or systems.
These restrictions make a server useless as a relay for clients that
do not support full SMTP functionality. Consequently, restricted-
capability clients MUST NOT assume that any SMTP server on the
Internet can be used as their mail processing (relaying) site. If a
RCPT command appears without a previous MAIL command, the server MUST
return a 503 "Bad sequence of commands" response. The optional
<rcpt-parameters> are associated with negotiated SMTP service
extensions (see Section 2.2).
Since it has been a common source of errors, it is worth noting that
spaces are not permitted on either side of the colon following FROM
in the MAIL command or TO in the RCPT command. The syntax is exactly
as given above.
The third step in the procedure is the DATA command (or some
alternative specified in a service extension).
DATA <CRLF>
If accepted, the SMTP server returns a 354 Intermediate reply and
considers all succeeding lines up to but not including the end of
mail data indicator to be the message text. When the end of text is
successfully received and stored, the SMTP-receiver sends a "250 OK"
reply.
Since the mail data is sent on the transmission channel, the end of
mail data must be indicated so that the command and reply dialog can
be resumed. SMTP indicates the end of the mail data by sending a
line containing only a "." (period or full stop). A transparency
procedure is used to prevent this from interfering with the user's
text (see Section 4.5.2).
The end of mail data indicator also confirms the mail transaction and
tells the SMTP server to now process the stored recipients and mail
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data. If accepted, the SMTP server returns a "250 OK" reply. The
DATA command can fail at only two points in the protocol exchange:
If there was no MAIL, or no RCPT, command, or all such commands were
rejected, the server MAY return a "command out of sequence" (503) or
"no valid recipients" (554) reply in response to the DATA command.
If one of those replies (or any other 5yz reply) is received, the
client MUST NOT send the message data; more generally, message data
MUST NOT be sent unless a 354 reply is received.
If the verb is initially accepted and the 354 reply issued, the DATA
command should fail only if the mail transaction was incomplete (for
example, no recipients), if resources were unavailable (including, of
course, the server unexpectedly becoming unavailable), or if the
server determines that the message should be rejected for policy or
other reasons.
However, in practice, some servers do not perform recipient
verification until after the message text is received. These servers
SHOULD treat a failure for one or more recipients as a "subsequent
failure" and return a mail message as discussed in Section 6 and, in
particular, in Section 6.1. Using a "550 mailbox not found" (or
equivalent) reply code after the data are accepted makes it difficult
or impossible for the client to determine which recipients failed.
When the RFC 822 format ([28], [4]) is being used, the mail data
include the header fields such as those named Date, Subject, To, Cc,
and From. Server SMTP systems SHOULD NOT reject messages based on
perceived defects in the RFC 822 or MIME (RFC 2045 [21]) message
header section or message body. In particular, they MUST NOT reject
messages in which the numbers of Resent-header fields do not match or
Resent-to appears without Resent-from and/or Resent-date.
Mail transaction commands MUST be used in the order discussed above.
3.4. Forwarding for Address Correction or Updating
Forwarding support is most often required to consolidate and simplify
addresses within, or relative to, some enterprise and less frequently
to establish addresses to link a person's prior address with a
current one. Silent forwarding of messages (without server
notification to the sender), for security or non-disclosure purposes,
is common in the contemporary Internet.
In both the enterprise and the "new address" cases, information
hiding (and sometimes security) considerations argue against exposure
of the "final" address through the SMTP protocol as a side effect of
the forwarding activity. This may be especially important when the
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final address may not even be reachable by the sender. Consequently,
the "forwarding" mechanisms described in Section 3.2 of RFC 821, and
especially the 251 (corrected destination) and 551 reply codes from
RCPT must be evaluated carefully by implementers and, when they are
available, by those configuring systems (see also Section 7.4).
In particular:
o Servers MAY forward messages when they are aware of an address
change. When they do so, they MAY either provide address-updating
information with a 251 code, or may forward "silently" and return
a 250 code. However, if a 251 code is used, they MUST NOT assume
that the client will actually update address information or even
return that information to the user.
Alternately,
o Servers MAY reject messages or return them as non-deliverable when
they cannot be delivered precisely as addressed. When they do so,
they MAY either provide address-updating information with a 551
code, or may reject the message as undeliverable with a 550 code
and no address-specific information. However, if a 551 code is
used, they MUST NOT assume that the client will actually update
address information or even return that information to the user.
SMTP server implementations that support the 251 and/or 551 reply
codes SHOULD provide configuration mechanisms so that sites that
conclude that they would undesirably disclose information can disable
or restrict their use.
3.5. Commands for Debugging Addresses
3.5.1. Overview
SMTP provides commands to verify a user name or obtain the content of
a mailing list. This is done with the VRFY and EXPN commands, which
have character string arguments. Implementations SHOULD support VRFY
and EXPN (however, see Section 3.5.2 and Section 7.3).
For the VRFY command, the string is a user name or a user name and
domain (see below). If a normal (i.e., 250) response is returned,
the response MAY include the full name of the user and MUST include
the mailbox of the user. It MUST be in either of the following
forms:
User Name <local-part@domain>
local-part@domain
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When a name that is the argument to VRFY could identify more than one
mailbox, the server MAY either note the ambiguity or identify the
alternatives. In other words, any of the following are legitimate
responses to VRFY:
553 User ambiguous
or
553- Ambiguous; Possibilities are
553-Joe Smith <jsmith@foo.com>
553-Harry Smith <hsmith@foo.com>
553 Melvin Smith <dweep@foo.com>
or
553-Ambiguous; Possibilities
553- <jsmith@foo.com>
553- <hsmith@foo.com>
553 <dweep@foo.com>
Under normal circumstances, a client receiving a 553 reply would be
expected to expose the result to the user. Use of exactly the forms
given, and the "user ambiguous" or "ambiguous" keywords, possibly
supplemented by extended reply codes, such as those described in RFC
3463 [25], will facilitate automated translation into other languages
as needed. Of course, a client that was highly automated or that was
operating in another language than English might choose to try to
translate the response to return some other indication to the user
than the literal text of the reply, or to take some automated action
such as consulting a directory service for additional information
before reporting to the user.
For the EXPN command, the string identifies a mailing list, and the
successful (i.e., 250) multiline response MAY include the full name
of the users and MUST give the mailboxes on the mailing list.
In some hosts, the distinction between a mailing list and an alias
for a single mailbox is a bit fuzzy, since a common data structure
may hold both types of entries, and it is possible to have mailing
lists containing only one mailbox. If a request is made to apply
VRFY to a mailing list, a positive response MAY be given if a message
so addressed would be delivered to everyone on the list, otherwise an
error SHOULD be reported (e.g., "550 That is a mailing list, not a
user" or "252 Unable to verify members of mailing list"). If a
request is made to expand a user name, the server MAY return a
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positive response consisting of a list containing one name, or an
error MAY be reported (e.g., "550 That is a user name, not a mailing
list").
In the case of a successful multiline reply (normal for EXPN),
exactly one mailbox is to be specified on each line of the reply.
The case of an ambiguous request is discussed above.
"User name" is a fuzzy term and has been used deliberately. An
implementation of the VRFY or EXPN commands MUST include at least
recognition of local mailboxes as "user names". However, since
current Internet practice often results in a single host handling
mail for multiple domains, hosts, especially hosts that provide this
functionality, SHOULD accept the "local-part@domain" form as a "user
name"; hosts MAY also choose to recognize other strings as "user
names".
The case of expanding a mailbox list requires a multiline reply, such
as:
C: EXPN Example-People
S: 250-Jon Postel <Postel@isi.edu>
S: 250-Fred Fonebone <Fonebone@physics.foo-u.edu>
S: 250 Sam Q. Smith <SQSmith@specific.generic.com>
or
C: EXPN Executive-Washroom-List
S: 550 Access Denied to You.
The character string arguments of the VRFY and EXPN commands cannot
be further restricted due to the variety of implementations of the
user name and mailbox list concepts. On some systems, it may be
appropriate for the argument of the EXPN command to be a file name
for a file containing a mailing list, but again there are a variety
of file naming conventions in the Internet. Similarly, historical
variations in what is returned by these commands are such that the
response SHOULD be interpreted very carefully, if at all, and SHOULD
generally only be used for diagnostic purposes.
3.5.2. VRFY Normal Response
When normal (2yz or 551) responses are returned from a VRFY or EXPN
request, the reply MUST include the <Mailbox> name using a
"<local-part@domain>" construction, where "domain" is a fully-
qualified domain name. In circumstances exceptional enough to
justify violating the intent of this specification, free-form text
MAY be returned. In order to facilitate parsing by both computers
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and people, addresses SHOULD appear in pointed brackets. When
addresses, rather than free-form debugging information, are returned,
EXPN and VRFY MUST return only valid domain addresses that are usable
in SMTP RCPT commands. Consequently, if an address implies delivery
to a program or other system, the mailbox name used to reach that
target MUST be given. Paths (explicit source routes) MUST NOT be
returned by VRFY or EXPN.
Server implementations SHOULD support both VRFY and EXPN. For
security reasons, implementations MAY provide local installations a
way to disable either or both of these commands through configuration
options or the equivalent (see Section 7.3). When these commands are
supported, they are not required to work across relays when relaying
is supported. Since they were both optional in RFC 821, but VRFY was
made mandatory in RFC 1123 [3], if EXPN is supported, it MUST be
listed as a service extension in an EHLO response. VRFY MAY be
listed as a convenience but, since support for it is required, SMTP
clients are not required to check for its presence on the extension
list before using it.
3.5.3. Meaning of VRFY or EXPN Success Response
A server MUST NOT return a 250 code in response to a VRFY or EXPN
command unless it has actually verified the address. In particular,
a server MUST NOT return 250 if all it has done is to verify that the
syntax given is valid. In that case, 502 (Command not implemented)
or 500 (Syntax error, command unrecognized) SHOULD be returned. As
stated elsewhere, implementation (in the sense of actually validating
addresses and returning information) of VRFY and EXPN are strongly
recommended. Hence, implementations that return 500 or 502 for VRFY
are not in full compliance with this specification.
There may be circumstances where an address appears to be valid but
cannot reasonably be verified in real time, particularly when a
server is acting as a mail exchanger for another server or domain.
"Apparent validity", in this case, would normally involve at least
syntax checking and might involve verification that any domains
specified were ones to which the host expected to be able to relay
mail. In these situations, reply code 252 SHOULD be returned. These
cases parallel the discussion of RCPT verification in Section 2.1.
Similarly, the discussion in Section 3.4 applies to the use of reply
codes 251 and 551 with VRFY (and EXPN) to indicate addresses that are
recognized but that would be forwarded or rejected were mail received
for them. Implementations generally SHOULD be more aggressive about
address verification in the case of VRFY than in the case of RCPT,
even if it takes a little longer to do so.
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3.5.4. Semantics and Applications of EXPN
EXPN is often very useful in debugging and understanding problems
with mailing lists and multiple-target-address aliases. Some systems
have attempted to use source expansion of mailing lists as a means of
eliminating duplicates. The propagation of aliasing systems with
mail on the Internet for hosts (typically with MX and CNAME DNS
records), for mailboxes (various types of local host aliases), and in
various proxying arrangements has made it nearly impossible for these
strategies to work consistently, and mail systems SHOULD NOT attempt
them.
3.6. Relaying and Mail Routing
3.6.1. Source Routes and Relaying
In general, the availability of Mail eXchanger records in the domain
name system (RFC 1035 [2], RFC 974 [12]) makes the use of explicit
source routes in the Internet mail system unnecessary. Many
historical problems with the interpretation of explicit source routes
have made their use undesirable. SMTP clients SHOULD NOT generate
explicit source routes except under unusual circumstances. SMTP
servers MAY decline to act as mail relays or to accept addresses that
specify source routes. When route information is encountered, SMTP
servers MAY ignore the route information and simply send to the final
destination specified as the last element in the route and SHOULD do
so. There has been an invalid practice of using names that do not
appear in the DNS as destination names, with the senders counting on
the intermediate hosts specified in source routing to resolve any
problems. If source routes are stripped, this practice will cause
failures. This is one of several reasons why SMTP clients MUST NOT
generate invalid source routes or depend on serial resolution of
names.
When source routes are not used, the process described in RFC 821 for
constructing a reverse-path from the forward-path is not applicable
and the reverse-path at the time of delivery will simply be the
address that appeared in the MAIL command.
3.6.2. Mail eXchange Records and Relaying
A relay SMTP server is usually the target of a DNS MX record that
designates it, rather than the final delivery system. The relay
server may accept or reject the task of relaying the mail in the same
way it accepts or rejects mail for a local user. If it accepts the
task, it then becomes an SMTP client, establishes a transmission
channel to the next SMTP server specified in the DNS (according to
the rules in Section 5), and sends it the mail. If it declines to
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relay mail to a particular address for policy reasons, a 550 response
SHOULD be returned.
This specification does not deal with the verification of return
paths for use in delivery notifications. Recent work, such as that
on SPF [29] and DKIM [30] [31], has been done to provide ways to
ascertain that an address is valid or belongs to the person who
actually sent the message. A server MAY attempt to verify the return
path before using its address for delivery notifications, but methods
of doing so are not defined here nor is any particular method
recommended at this time.
3.6.3. Message Submission Servers as Relays
Many mail-sending clients exist, especially in conjunction with
facilities that receive mail via POP3 or IMAP, that have limited
capability to support some of the requirements of this specification,
such as the ability to queue messages for subsequent delivery
attempts. For these clients, it is common practice to make private
arrangements to send all messages to a single server for processing
and subsequent distribution. SMTP, as specified here, is not ideally
suited for this role. A standardized mail submission protocol has
been developed that is gradually superseding practices based on SMTP
(see RFC 4409 [18]). In any event, because these arrangements are
private and fall outside the scope of this specification, they are
not described here.
It is important to note that MX records can point to SMTP servers
that act as gateways into other environments, not just SMTP relays
and final delivery systems; see Sections 3.7 and 5.
If an SMTP server has accepted the task of relaying the mail and
later finds that the destination is incorrect or that the mail cannot
be delivered for some other reason, then it MUST construct an
"undeliverable mail" notification message and send it to the
originator of the undeliverable mail (as indicated by the reverse-
path). Formats specified for non-delivery reports by other standards
(see, for example, RFC 3461 [32] and RFC 3464 [33]) SHOULD be used if
possible.
This notification message must be from the SMTP server at the relay
host or the host that first determines that delivery cannot be
accomplished. Of course, SMTP servers MUST NOT send notification
messages about problems transporting notification messages. One way
to prevent loops in error reporting is to specify a null reverse-path
in the MAIL command of a notification message. When such a message
is transmitted, the reverse-path MUST be set to null (see
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Section 4.5.5 for additional discussion). A MAIL command with a null
reverse-path appears as follows:
MAIL FROM:<>
As discussed in Section 6.4, a relay SMTP has no need to inspect or
act upon the header section or body of the message data and MUST NOT
do so except to add its own "Received:" header field (Section 4.4)
and, optionally, to attempt to detect looping in the mail system (see
Section 6.3). Of course, this prohibition also applies to any
modifications of these header fields or text (see also Section 7.9).
3.7. Mail Gatewaying
While the relay function discussed above operates within the Internet
SMTP transport service environment, MX records or various forms of
explicit routing may require that an intermediate SMTP server perform
a translation function between one transport service and another. As
discussed in Section 2.3.10, when such a system is at the boundary
between two transport service environments, we refer to it as a
"gateway" or "gateway SMTP".
Gatewaying mail between different mail environments, such as
different mail formats and protocols, is complex and does not easily
yield to standardization. However, some general requirements may be
given for a gateway between the Internet and another mail
environment.
3.7.1. Header Fields in Gatewaying
Header fields MAY be rewritten when necessary as messages are
gatewayed across mail environment boundaries. This may involve
inspecting the message body or interpreting the local-part of the
destination address in spite of the prohibitions in Section 6.4.
Other mail systems gatewayed to the Internet often use a subset of
the RFC 822 header section or provide similar functionality with a
different syntax, but some of these mail systems do not have an
equivalent to the SMTP envelope. Therefore, when a message leaves
the Internet environment, it may be necessary to fold the SMTP
envelope information into the message header section. A possible
solution would be to create new header fields to carry the envelope
information (e.g., "X-SMTP-MAIL:" and "X-SMTP-RCPT:"); however, this
would require changes in mail programs in foreign environments and
might risk disclosure of private information (see Section 7.2).
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3.7.2. Received Lines in Gatewaying
When forwarding a message into or out of the Internet environment, a
gateway MUST prepend a Received: line, but it MUST NOT alter in any
way a Received: line that is already in the header section.
"Received:" header fields of messages originating from other
environments may not conform exactly to this specification. However,
the most important use of Received: lines is for debugging mail
faults, and this debugging can be severely hampered by well-meaning
gateways that try to "fix" a Received: line. As another consequence
of trace header fields arising in non-SMTP environments, receiving
systems MUST NOT reject mail based on the format of a trace header
field and SHOULD be extremely robust in the light of unexpected
information or formats in those header fields.
The gateway SHOULD indicate the environment and protocol in the "via"
clauses of Received header field(s) that it supplies.
3.7.3. Addresses in Gatewaying
From the Internet side, the gateway SHOULD accept all valid address
formats in SMTP commands and in the RFC 822 header section, and all
valid RFC 822 messages. Addresses and header fields generated by
gateways MUST conform to applicable standards (including this one and
RFC 5322 [4]). Gateways are, of course, subject to the same rules
for handling source routes as those described for other SMTP systems
in Section 3.3.
3.7.4. Other Header Fields in Gatewaying
The gateway MUST ensure that all header fields of a message that it
forwards into the Internet mail environment meet the requirements for
Internet mail. In particular, all addresses in "From:", "To:",
"Cc:", etc., header fields MUST be transformed (if necessary) to
satisfy the standard header syntax of RFC 5322 [4], MUST reference
only fully-qualified domain names, and MUST be effective and useful
for sending replies. The translation algorithm used to convert mail
from the Internet protocols to another environment's protocol SHOULD
ensure that error messages from the foreign mail environment are
delivered to the reverse-path from the SMTP envelope, not to an
address in the "From:", "Sender:", or similar header fields of the
message.
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3.7.5. Envelopes in Gatewaying
Similarly, when forwarding a message from another environment into
the Internet, the gateway SHOULD set the envelope return path in
accordance with an error message return address, if supplied by the
foreign environment. If the foreign environment has no equivalent
concept, the gateway must select and use a best approximation, with
the message originator's address as the default of last resort.
3.8. Terminating Sessions and Connections
An SMTP connection is terminated when the client sends a QUIT
command. The server responds with a positive reply code, after which
it closes the connection.
An SMTP server MUST NOT intentionally close the connection under
normal operational circumstances (see Section 7.8) except:
o After receiving a QUIT command and responding with a 221 reply.
o After detecting the need to shut down the SMTP service and
returning a 421 response code. This response code can be issued
after the server receives any command or, if necessary,
asynchronously from command receipt (on the assumption that the
client will receive it after the next command is issued).
o After a timeout, as specified in Section 4.5.3.2, occurs waiting
for the client to send a command or data.
In particular, a server that closes connections in response to
commands that are not understood is in violation of this
specification. Servers are expected to be tolerant of unknown
commands, issuing a 500 reply and awaiting further instructions from
the client.
An SMTP server that is forcibly shut down via external means SHOULD
attempt to send a line containing a 421 response code to the SMTP
client before exiting. The SMTP client will normally read the 421
response code after sending its next command.
SMTP clients that experience a connection close, reset, or other
communications failure due to circumstances not under their control
(in violation of the intent of this specification but sometimes
unavoidable) SHOULD, to maintain the robustness of the mail system,
treat the mail transaction as if a 451 response had been received and
act accordingly.
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3.9. Mailing Lists and Aliases
An SMTP-capable host SHOULD support both the alias and the list
models of address expansion for multiple delivery. When a message is
delivered or forwarded to each address of an expanded list form, the
return address in the envelope ("MAIL FROM:") MUST be changed to be
the address of a person or other entity who administers the list.
However, in this case, the message header section (RFC 5322 [4]) MUST
be left unchanged; in particular, the "From" field of the header
section is unaffected.
An important mail facility is a mechanism for multi-destination
delivery of a single message, by transforming (or "expanding" or
"exploding") a pseudo-mailbox address into a list of destination
mailbox addresses. When a message is sent to such a pseudo-mailbox
(sometimes called an "exploder"), copies are forwarded or
redistributed to each mailbox in the expanded list. Servers SHOULD
simply utilize the addresses on the list; application of heuristics
or other matching rules to eliminate some addresses, such as that of
the originator, is strongly discouraged. We classify such a pseudo-
mailbox as an "alias" or a "list", depending upon the expansion
rules.
3.9.1. Alias
To expand an alias, the recipient mailer simply replaces the pseudo-
mailbox address in the envelope with each of the expanded addresses
in turn; the rest of the envelope and the message body are left
unchanged. The message is then delivered or forwarded to each
expanded address.
3.9.2. List
A mailing list may be said to operate by "redistribution" rather than
by "forwarding". To expand a list, the recipient mailer replaces the
pseudo-mailbox address in the envelope with each of the expanded
addresses in turn. The return (backward-pointing) address in the
envelope is changed so that all error messages generated by the final
deliveries will be returned to a list administrator, not to the
message originator, who generally has no control over the contents of
the list and will typically find error messages annoying. Note that
the key difference between handling aliases (Section 3.9.1) and
forwarding (this subsection) is the change to the backward-pointing
address in this case. When a list constrains its processing to the
very limited set of modifications and actions described here, it is
attempting to emulate an MTA; such lists can be treated as a
continuation in email transit.
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There exist mailing lists that perform additional, sometimes
extensive, modifications to a message and its envelope. Such mailing
lists need to be viewed as full MUAs, which accept a delivery and
post a new message.
4. The SMTP Specifications
4.1. SMTP Commands
4.1.1. Command Semantics and Syntax
The SMTP commands define the mail transfer or the mail system
function requested by the user. SMTP commands are character strings
terminated by <CRLF>. The commands themselves are alphabetic
characters terminated by <SP> if parameters follow and <CRLF>
otherwise. (In the interest of improved interoperability, SMTP
receivers SHOULD tolerate trailing white space before the terminating
<CRLF>.) The syntax of the local part of a mailbox MUST conform to
receiver site conventions and the syntax specified in Section 4.1.2.
The SMTP commands are discussed below. The SMTP replies are
discussed in Section 4.2.
A mail transaction involves several data objects that are
communicated as arguments to different commands. The reverse-path is
the argument of the MAIL command, the forward-path is the argument of
the RCPT command, and the mail data is the argument of the DATA
command. These arguments or data objects must be transmitted and
held, pending the confirmation communicated by the end of mail data
indication that finalizes the transaction. The model for this is
that distinct buffers are provided to hold the types of data objects;
that is, there is a reverse-path buffer, a forward-path buffer, and a
mail data buffer. Specific commands cause information to be appended
to a specific buffer, or cause one or more buffers to be cleared.
Several commands (RSET, DATA, QUIT) are specified as not permitting
parameters. In the absence of specific extensions offered by the
server and accepted by the client, clients MUST NOT send such
parameters and servers SHOULD reject commands containing them as
having invalid syntax.
4.1.1.1. Extended HELLO (EHLO) or HELLO (HELO)
These commands are used to identify the SMTP client to the SMTP
server. The argument clause contains the fully-qualified domain name
of the SMTP client, if one is available. In situations in which the
SMTP client system does not have a meaningful domain name (e.g., when
its address is dynamically allocated and no reverse mapping record is
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available), the client SHOULD send an address literal (see
Section 4.1.3).
RFC 2821, and some earlier informal practices, encouraged following
the literal by information that would help to identify the client
system. That convention was not widely supported, and many SMTP
servers considered it an error. In the interest of interoperability,
it is probably wise for servers to be prepared for this string to
occur, but SMTP clients SHOULD NOT send it.
The SMTP server identifies itself to the SMTP client in the
connection greeting reply and in the response to this command.
A client SMTP SHOULD start an SMTP session by issuing the EHLO
command. If the SMTP server supports the SMTP service extensions, it
will give a successful response, a failure response, or an error
response. If the SMTP server, in violation of this specification,
does not support any SMTP service extensions, it will generate an
error response. Older client SMTP systems MAY, as discussed above,
use HELO (as specified in RFC 821) instead of EHLO, and servers MUST
support the HELO command and reply properly to it. In any event, a
client MUST issue HELO or EHLO before starting a mail transaction.
These commands, and a "250 OK" reply to one of them, confirm that
both the SMTP client and the SMTP server are in the initial state,
that is, there is no transaction in progress and all state tables and
buffers are cleared.
Syntax:
ehlo = "EHLO" SP ( Domain / address-literal ) CRLF
helo = "HELO" SP Domain CRLF
Normally, the response to EHLO will be a multiline reply. Each line
of the response contains a keyword and, optionally, one or more
parameters. Following the normal syntax for multiline replies, these
keywords follow the code (250) and a hyphen for all but the last
line, and the code and a space for the last line. The syntax for a
positive response, using the ABNF notation and terminal symbols of
RFC 5234 [7], is:
ehlo-ok-rsp = ( "250" SP Domain [ SP ehlo-greet ] CRLF )
/ ( "250-" Domain [ SP ehlo-greet ] CRLF
*( "250-" ehlo-line CRLF )
"250" SP ehlo-line CRLF )
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ehlo-greet = 1*(%d0-9 / %d11-12 / %d14-127)
; string of any characters other than CR or LF
ehlo-line = ehlo-keyword *( SP ehlo-param )
ehlo-keyword = (ALPHA / DIGIT) *(ALPHA / DIGIT / "-")
; additional syntax of ehlo-params depends on
; ehlo-keyword
ehlo-param = 1*(%d33-126)
; any CHAR excluding <SP> and all
; control characters (US-ASCII 0-31 and 127
; inclusive)
Although EHLO keywords may be specified in upper, lower, or mixed
case, they MUST always be recognized and processed in a case-
insensitive manner. This is simply an extension of practices
specified in RFC 821 and Section 2.4.
The EHLO response MUST contain keywords (and associated parameters if
required) for all commands not listed as "required" in Section 4.5.1
excepting only private-use commands as described in Section 4.1.5.
Private-use commands MAY be listed.
4.1.1.2. MAIL (MAIL)
This command is used to initiate a mail transaction in which the mail
data is delivered to an SMTP server that may, in turn, deliver it to
one or more mailboxes or pass it on to another system (possibly using
SMTP). The argument clause contains a reverse-path and may contain
optional parameters. In general, the MAIL command may be sent only
when no mail transaction is in progress, see Section 4.1.4.
The reverse-path consists of the sender mailbox. Historically, that
mailbox might optionally have been preceded by a list of hosts, but
that behavior is now deprecated (see Appendix C). In some types of
reporting messages for which a reply is likely to cause a mail loop
(for example, mail delivery and non-delivery notifications), the
reverse-path may be null (see Section 3.6).
This command clears the reverse-path buffer, the forward-path buffer,
and the mail data buffer, and it inserts the reverse-path information
from its argument clause into the reverse-path buffer.
If service extensions were negotiated, the MAIL command may also
carry parameters associated with a particular service extension.
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Syntax:
mail = "MAIL FROM:" Reverse-path
[SP Mail-parameters] CRLF
4.1.1.3. RECIPIENT (RCPT)
This command is used to identify an individual recipient of the mail
data; multiple recipients are specified by multiple uses of this
command. The argument clause contains a forward-path and may contain
optional parameters.
The forward-path normally consists of the required destination
mailbox. Sending systems SHOULD NOT generate the optional list of
hosts known as a source route. Receiving systems MUST recognize
source route syntax but SHOULD strip off the source route
specification and utilize the domain name associated with the mailbox
as if the source route had not been provided.
Similarly, relay hosts SHOULD strip or ignore source routes, and
names MUST NOT be copied into the reverse-path. When mail reaches
its ultimate destination (the forward-path contains only a
destination mailbox), the SMTP server inserts it into the destination
mailbox in accordance with its host mail conventions.
This command appends its forward-path argument to the forward-path
buffer; it does not change the reverse-path buffer nor the mail data
buffer.
For example, mail received at relay host xyz.com with envelope
commands
MAIL FROM:<userx@y.foo.org>
RCPT TO:<@hosta.int,@jkl.org:userc@d.bar.org>
will normally be sent directly on to host d.bar.org with envelope
commands
MAIL FROM:<userx@y.foo.org>
RCPT TO:<userc@d.bar.org>
As provided in Appendix C, xyz.com MAY also choose to relay the
message to hosta.int, using the envelope commands
MAIL FROM:<userx@y.foo.org>
RCPT TO:<@hosta.int,@jkl.org:userc@d.bar.org>
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or to jkl.org, using the envelope commands
MAIL FROM:<userx@y.foo.org>
RCPT TO:<@jkl.org:userc@d.bar.org>
Attempting to use relaying this way is now strongly discouraged.
Since hosts are not required to relay mail at all, xyz.com MAY also
reject the message entirely when the RCPT command is received, using
a 550 code (since this is a "policy reason").
If service extensions were negotiated, the RCPT command may also
carry parameters associated with a particular service extension
offered by the server. The client MUST NOT transmit parameters other
than those associated with a service extension offered by the server
in its EHLO response.
Syntax:
rcpt = "RCPT TO:" ( "<Postmaster@" Domain ">" / "<Postmaster>" /
Forward-path ) [SP Rcpt-parameters] CRLF
Note that, in a departure from the usual rules for
local-parts, the "Postmaster" string shown above is
treated as case-insensitive.
4.1.1.4. DATA (DATA)
The receiver normally sends a 354 response to DATA, and then treats
the lines (strings ending in <CRLF> sequences, as described in
Section 2.3.7) following the command as mail data from the sender.
This command causes the mail data to be appended to the mail data
buffer. The mail data may contain any of the 128 ASCII character
codes, although experience has indicated that use of control
characters other than SP, HT, CR, and LF may cause problems and
SHOULD be avoided when possible.
The mail data are terminated by a line containing only a period, that
is, the character sequence "<CRLF>.<CRLF>", where the first <CRLF> is
actually the terminator of the previous line (see Section 4.5.2).
This is the end of mail data indication. The first <CRLF> of this
terminating sequence is also the <CRLF> that ends the final line of
the data (message text) or, if there was no mail data, ends the DATA
command itself (the "no mail data" case does not conform to this
specification since it would require that neither the trace header
fields required by this specification nor the message header section
required by RFC 5322 [4] be transmitted). An extra <CRLF> MUST NOT
be added, as that would cause an empty line to be added to the
message. The only exception to this rule would arise if the message
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body were passed to the originating SMTP-sender with a final "line"
that did not end in <CRLF>; in that case, the originating SMTP system
MUST either reject the message as invalid or add <CRLF> in order to
have the receiving SMTP server recognize the "end of data" condition.
The custom of accepting lines ending only in <LF>, as a concession to
non-conforming behavior on the part of some UNIX systems, has proven
to cause more interoperability problems than it solves, and SMTP
server systems MUST NOT do this, even in the name of improved
robustness. In particular, the sequence "<LF>.<LF>" (bare line
feeds, without carriage returns) MUST NOT be treated as equivalent to
<CRLF>.<CRLF> as the end of mail data indication.
Receipt of the end of mail data indication requires the server to
process the stored mail transaction information. This processing
consumes the information in the reverse-path buffer, the forward-path
buffer, and the mail data buffer, and on the completion of this
command these buffers are cleared. If the processing is successful,
the receiver MUST send an OK reply. If the processing fails, the
receiver MUST send a failure reply. The SMTP model does not allow
for partial failures at this point: either the message is accepted by
the server for delivery and a positive response is returned or it is
not accepted and a failure reply is returned. In sending a positive
"250 OK" completion reply to the end of data indication, the receiver
takes full responsibility for the message (see Section 6.1). Errors
that are diagnosed subsequently MUST be reported in a mail message,
as discussed in Section 4.4.
When the SMTP server accepts a message either for relaying or for
final delivery, it inserts a trace record (also referred to
interchangeably as a "time stamp line" or "Received" line) at the top
of the mail data. This trace record indicates the identity of the
host that sent the message, the identity of the host that received
the message (and is inserting this time stamp), and the date and time
the message was received. Relayed messages will have multiple time
stamp lines. Details for formation of these lines, including their
syntax, is specified in Section 4.4.
Additional discussion about the operation of the DATA command appears
in Section 3.3.
Syntax:
data = "DATA" CRLF
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4.1.1.5. RESET (RSET)
This command specifies that the current mail transaction will be
aborted. Any stored sender, recipients, and mail data MUST be
discarded, and all buffers and state tables cleared. The receiver
MUST send a "250 OK" reply to a RSET command with no arguments. A
reset command may be issued by the client at any time. It is
effectively equivalent to a NOOP (i.e., it has no effect) if issued
immediately after EHLO, before EHLO is issued in the session, after
an end of data indicator has been sent and acknowledged, or
immediately before a QUIT. An SMTP server MUST NOT close the
connection as the result of receiving a RSET; that action is reserved
for QUIT (see Section 4.1.1.10).
Since EHLO implies some additional processing and response by the
server, RSET will normally be more efficient than reissuing that
command, even though the formal semantics are the same.
There are circumstances, contrary to the intent of this
specification, in which an SMTP server may receive an indication that
the underlying TCP connection has been closed or reset. To preserve
the robustness of the mail system, SMTP servers SHOULD be prepared
for this condition and SHOULD treat it as if a QUIT had been received
before the connection disappeared.
Syntax:
rset = "RSET" CRLF
4.1.1.6. VERIFY (VRFY)
This command asks the receiver to confirm that the argument
identifies a user or mailbox. If it is a user name, information is
returned as specified in Section 3.5.
This command has no effect on the reverse-path buffer, the forward-
path buffer, or the mail data buffer.
Syntax:
vrfy = "VRFY" SP String CRLF
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4.1.1.7. EXPAND (EXPN)
This command asks the receiver to confirm that the argument
identifies a mailing list, and if so, to return the membership of
that list. If the command is successful, a reply is returned
containing information as described in Section 3.5. This reply will
have multiple lines except in the trivial case of a one-member list.
This command has no effect on the reverse-path buffer, the forward-
path buffer, or the mail data buffer, and it may be issued at any
time.
Syntax:
expn = "EXPN" SP String CRLF
4.1.1.8. HELP (HELP)
This command causes the server to send helpful information to the
client. The command MAY take an argument (e.g., any command name)
and return more specific information as a response.
This command has no effect on the reverse-path buffer, the forward-
path buffer, or the mail data buffer, and it may be issued at any
time.
SMTP servers SHOULD support HELP without arguments and MAY support it
with arguments.
Syntax:
help = "HELP" [ SP String ] CRLF
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4.1.1.9. NOOP (NOOP)
This command does not affect any parameters or previously entered
commands. It specifies no action other than that the receiver send a
"250 OK" reply.
This command has no effect on the reverse-path buffer, the forward-
path buffer, or the mail data buffer, and it may be issued at any
time. If a parameter string is specified, servers SHOULD ignore it.
Syntax:
noop = "NOOP" [ SP String ] CRLF
4.1.1.10. QUIT (QUIT)
This command specifies that the receiver MUST send a "221 OK" reply,
and then close the transmission channel.
The receiver MUST NOT intentionally close the transmission channel
until it receives and replies to a QUIT command (even if there was an
error). The sender MUST NOT intentionally close the transmission
channel until it sends a QUIT command, and it SHOULD wait until it
receives the reply (even if there was an error response to a previous
command). If the connection is closed prematurely due to violations
of the above or system or network failure, the server MUST cancel any
pending transaction, but not undo any previously completed
transaction, and generally MUST act as if the command or transaction
in progress had received a temporary error (i.e., a 4yz response).
The QUIT command may be issued at any time. Any current uncompleted
mail transaction will be aborted.
Syntax:
quit = "QUIT" CRLF
4.1.1.11. Mail-Parameter and Rcpt-Parameter Error Responses
If the server SMTP does not recognize or cannot implement one or more
of the parameters associated with a particular MAIL FROM or RCPT TO
command, it will return code 555.
If, for some reason, the server is temporarily unable to accommodate
one or more of the parameters associated with a MAIL FROM or RCPT TO
command, and if the definition of the specific parameter does not
mandate the use of another code, it should return code 455.
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Errors specific to particular parameters and their values will be
specified in the parameter's defining RFC.
4.1.2. Command Argument Syntax
The syntax of the argument clauses of the above commands (using the
syntax specified in RFC 5234 [7] where applicable) is given below.
Some of the productions given below are used only in conjunction with
source routes as described in Appendix C. Terminals not defined in
this document, such as ALPHA, DIGIT, SP, CR, LF, CRLF, are as defined
in the "core" syntax in Section 6 of RFC 5234 [7] or in the message
format syntax in RFC 5322 [4].
Reverse-path = Path / "<>"
Forward-path = Path
Path = "<" [ A-d-l ":" ] Mailbox ">"
A-d-l = At-domain *( "," At-domain )
; Note that this form, the so-called "source
; route", MUST BE accepted, SHOULD NOT be
; generated, and SHOULD be ignored.
At-domain = "@" Domain
Mail-parameters = esmtp-param *(SP esmtp-param)
Rcpt-parameters = esmtp-param *(SP esmtp-param)
esmtp-param = esmtp-keyword ["=" esmtp-value]
esmtp-keyword = (ALPHA / DIGIT) *(ALPHA / DIGIT / "-")
esmtp-value = 1*(%d33-60 / %d62-126)
; any CHAR excluding "=", SP, and control
; characters. If this string is an email address,
; i.e., a Mailbox, then the "xtext" syntax [32]
; SHOULD be used.
Keyword = Ldh-str
Argument = Atom
Domain = sub-domain *("." sub-domain)
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sub-domain = Let-dig [Ldh-str]
Let-dig = ALPHA / DIGIT
Ldh-str = *( ALPHA / DIGIT / "-" ) Let-dig
address-literal = "[" ( IPv4-address-literal /
IPv6-address-literal /
General-address-literal ) "]"
; See Section 4.1.3
Mailbox = Local-part "@" ( Domain / address-literal )
Local-part = Dot-string / Quoted-string
; MAY be case-sensitive
Dot-string = Atom *("." Atom)
Atom = 1*atext
Quoted-string = DQUOTE *QcontentSMTP DQUOTE
QcontentSMTP = qtextSMTP / quoted-pairSMTP
quoted-pairSMTP = %d92 %d32-126
; i.e., backslash followed by any ASCII
; graphic (including itself) or SPace
qtextSMTP = %d32-33 / %d35-91 / %d93-126
; i.e., within a quoted string, any
; ASCII graphic or space is permitted
; without blackslash-quoting except
; double-quote and the backslash itself.
String = Atom / Quoted-string
While the above definition for Local-part is relatively permissive,
for maximum interoperability, a host that expects to receive mail
SHOULD avoid defining mailboxes where the Local-part requires (or
uses) the Quoted-string form or where the Local-part is case-
sensitive. For any purposes that require generating or comparing
Local-parts (e.g., to specific mailbox names), all quoted forms MUST
be treated as equivalent, and the sending system SHOULD transmit the
form that uses the minimum quoting possible.
Systems MUST NOT define mailboxes in such a way as to require the use
in SMTP of non-ASCII characters (octets with the high order bit set
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to one) or ASCII "control characters" (decimal value 0-31 and 127).
These characters MUST NOT be used in MAIL or RCPT commands or other
commands that require mailbox names.
Note that the backslash, "\", is a quote character, which is used to
indicate that the next character is to be used literally (instead of
its normal interpretation). For example, "Joe\,Smith" indicates a
single nine-character user name string with the comma being the
fourth character of that string.
To promote interoperability and consistent with long-standing
guidance about conservative use of the DNS in naming and applications
(e.g., see Section 2.3.1 of the base DNS document, RFC 1035 [2]),
characters outside the set of alphabetic characters, digits, and
hyphen MUST NOT appear in domain name labels for SMTP clients or
servers. In particular, the underscore character is not permitted.
SMTP servers that receive a command in which invalid character codes
have been employed, and for which there are no other reasons for
rejection, MUST reject that command with a 501 response (this rule,
like others, could be overridden by appropriate SMTP extensions).
4.1.3. Address Literals
Sometimes a host is not known to the domain name system and
communication (and, in particular, communication to report and repair
the error) is blocked. To bypass this barrier, a special literal
form of the address is allowed as an alternative to a domain name.
For IPv4 addresses, this form uses four small decimal integers
separated by dots and enclosed by brackets such as [123.255.37.2],
which indicates an (IPv4) Internet Address in sequence-of-octets
form. For IPv6 and other forms of addressing that might eventually
be standardized, the form consists of a standardized "tag" that
identifies the address syntax, a colon, and the address itself, in a
format specified as part of the relevant standards (i.e., RFC 4291
[8] for IPv6).
Specifically:
IPv4-address-literal = Snum 3("." Snum)
IPv6-address-literal = "IPv6:" IPv6-addr
General-address-literal = Standardized-tag ":" 1*dcontent
Standardized-tag = Ldh-str
; Standardized-tag MUST be specified in a
; Standards-Track RFC and registered with IANA
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dcontent = %d33-90 / ; Printable US-ASCII
%d94-126 ; excl. "[", "\", "]"
Snum = 1*3DIGIT
; representing a decimal integer
; value in the range 0 through 255
IPv6-addr = IPv6-full / IPv6-comp / IPv6v4-full / IPv6v4-comp
IPv6-hex = 1*4HEXDIG
IPv6-full = IPv6-hex 7(":" IPv6-hex)
IPv6-comp = [IPv6-hex *5(":" IPv6-hex)] "::"
[IPv6-hex *5(":" IPv6-hex)]
; The "::" represents at least 2 16-bit groups of
; zeros. No more than 6 groups in addition to the
; "::" may be present.
IPv6v4-full = IPv6-hex 5(":" IPv6-hex) ":" IPv4-address-literal
IPv6v4-comp = [IPv6-hex *3(":" IPv6-hex)] "::"
[IPv6-hex *3(":" IPv6-hex) ":"]
IPv4-address-literal
; The "::" represents at least 2 16-bit groups of
; zeros. No more than 4 groups in addition to the
; "::" and IPv4-address-literal may be present.
4.1.4. Order of Commands
There are restrictions on the order in which these commands may be
used.
A session that will contain mail transactions MUST first be
initialized by the use of the EHLO command. An SMTP server SHOULD
accept commands for non-mail transactions (e.g., VRFY or EXPN)
without this initialization.
An EHLO command MAY be issued by a client later in the session. If
it is issued after the session begins and the EHLO command is
acceptable to the SMTP server, the SMTP server MUST clear all buffers
and reset the state exactly as if a RSET command had been issued. In
other words, the sequence of RSET followed immediately by EHLO is
redundant, but not harmful other than in the performance cost of
executing unnecessary commands.
If the EHLO command is not acceptable to the SMTP server, 501, 500,
502, or 550 failure replies MUST be returned as appropriate. The
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SMTP server MUST stay in the same state after transmitting these
replies that it was in before the EHLO was received.
The SMTP client MUST, if possible, ensure that the domain parameter
to the EHLO command is a primary host name as specified for this
command in Section 2.3.5. If this is not possible (e.g., when the
client's address is dynamically assigned and the client does not have
an obvious name), an address literal SHOULD be substituted for the
domain name.
An SMTP server MAY verify that the domain name argument in the EHLO
command actually corresponds to the IP address of the client.
However, if the verification fails, the server MUST NOT refuse to
accept a message on that basis. Information captured in the
verification attempt is for logging and tracing purposes. Note that
this prohibition applies to the matching of the parameter to its IP
address only; see Section 7.9 for a more extensive discussion of
rejecting incoming connections or mail messages.
The NOOP, HELP, EXPN, VRFY, and RSET commands can be used at any time
during a session, or without previously initializing a session. SMTP
servers SHOULD process these normally (that is, not return a 503
code) even if no EHLO command has yet been received; clients SHOULD
open a session with EHLO before sending these commands.
If these rules are followed, the example in RFC 821 that shows "550
access denied to you" in response to an EXPN command is incorrect
unless an EHLO command precedes the EXPN or the denial of access is
based on the client's IP address or other authentication or
authorization-determining mechanisms.
The MAIL command (or the obsolete SEND, SOML, or SAML commands)
begins a mail transaction. Once started, a mail transaction consists
of a transaction beginning command, one or more RCPT commands, and a
DATA command, in that order. A mail transaction may be aborted by
the RSET, a new EHLO, or the QUIT command. There may be zero or more
transactions in a session. MAIL (or SEND, SOML, or SAML) MUST NOT be
sent if a mail transaction is already open, i.e., it should be sent
only if no mail transaction had been started in the session, or if
the previous one successfully concluded with a successful DATA
command, or if the previous one was aborted, e.g., with a RSET or new
EHLO.
If the transaction beginning command argument is not acceptable, a
501 failure reply MUST be returned and the SMTP server MUST stay in
the same state. If the commands in a transaction are out of order to
the degree that they cannot be processed by the server, a 503 failure
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reply MUST be returned and the SMTP server MUST stay in the same
state.
The last command in a session MUST be the QUIT command. The QUIT
command SHOULD be used by the client SMTP to request connection
closure, even when no session opening command was sent and accepted.
4.1.5. Private-Use Commands
As specified in Section 2.2.2, commands starting in "X" may be used
by bilateral agreement between the client (sending) and server
(receiving) SMTP agents. An SMTP server that does not recognize such
a command is expected to reply with "500 Command not recognized". An
extended SMTP server MAY list the feature names associated with these
private commands in the response to the EHLO command.
Commands sent or accepted by SMTP systems that do not start with "X"
MUST conform to the requirements of Section 2.2.2.
4.2. SMTP Replies
Replies to SMTP commands serve to ensure the synchronization of
requests and actions in the process of mail transfer and to guarantee
that the SMTP client always knows the state of the SMTP server.
Every command MUST generate exactly one reply.
The details of the command-reply sequence are described in
Section 4.3.
An SMTP reply consists of a three digit number (transmitted as three
numeric characters) followed by some text unless specified otherwise
in this document. The number is for use by automata to determine
what state to enter next; the text is for the human user. The three
digits contain enough encoded information that the SMTP client need
not examine the text and may either discard it or pass it on to the
user, as appropriate. Exceptions are as noted elsewhere in this
document. In particular, the 220, 221, 251, 421, and 551 reply codes
are associated with message text that must be parsed and interpreted
by machines. In the general case, the text may be receiver dependent
and context dependent, so there are likely to be varying texts for
each reply code. A discussion of the theory of reply codes is given
in Section 4.2.1. Formally, a reply is defined to be the sequence: a
three-digit code, <SP>, one line of text, and <CRLF>, or a multiline
reply (as defined in the same section). Since, in violation of this
specification, the text is sometimes not sent, clients that do not
receive it SHOULD be prepared to process the code alone (with or
without a trailing space character). Only the EHLO, EXPN, and HELP
commands are expected to result in multiline replies in normal
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circumstances; however, multiline replies are allowed for any
command.
In ABNF, server responses are:
Greeting = ( "220 " (Domain / address-literal)
[ SP textstring ] CRLF ) /
( "220-" (Domain / address-literal)
[ SP textstring ] CRLF
*( "220-" [ textstring ] CRLF )
"220" [ SP textstring ] CRLF )
textstring = 1*(%d09 / %d32-126) ; HT, SP, Printable US-ASCII
Reply-line = *( Reply-code "-" [ textstring ] CRLF )
Reply-code [ SP textstring ] CRLF
Reply-code = %x32-35 %x30-35 %x30-39
where "Greeting" appears only in the 220 response that announces that
the server is opening its part of the connection. (Other possible
server responses upon connection follow the syntax of Reply-line.)
An SMTP server SHOULD send only the reply codes listed in this
document. An SMTP server SHOULD use the text shown in the examples
whenever appropriate.
An SMTP client MUST determine its actions only by the reply code, not
by the text (except for the "change of address" 251 and 551 and, if
necessary, 220, 221, and 421 replies); in the general case, any text,
including no text at all (although senders SHOULD NOT send bare
codes), MUST be acceptable. The space (blank) following the reply
code is considered part of the text. Whenever possible, a receiver-
SMTP SHOULD test the first digit (severity indication) of the reply
code.
The list of codes that appears below MUST NOT be construed as
permanent. While the addition of new codes should be a rare and
significant activity, with supplemental information in the textual
part of the response being preferred, new codes may be added as the
result of new Standards or Standards-Track specifications.
Consequently, a sender-SMTP MUST be prepared to handle codes not
specified in this document and MUST do so by interpreting the first
digit only.
In the absence of extensions negotiated with the client, SMTP servers
MUST NOT send reply codes whose first digits are other than 2, 3, 4,
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or 5. Clients that receive such out-of-range codes SHOULD normally
treat them as fatal errors and terminate the mail transaction.
4.2.1. Reply Code Severities and Theory
The three digits of the reply each have a special significance. The
first digit denotes whether the response is good, bad, or incomplete.
An unsophisticated SMTP client, or one that receives an unexpected
code, will be able to determine its next action (proceed as planned,
redo, retrench, etc.) by examining this first digit. An SMTP client
that wants to know approximately what kind of error occurred (e.g.,
mail system error, command syntax error) may examine the second
digit. The third digit and any supplemental information that may be
present is reserved for the finest gradation of information.
There are four values for the first digit of the reply code:
2yz Positive Completion reply
The requested action has been successfully completed. A new
request may be initiated.
3yz Positive Intermediate reply
The command has been accepted, but the requested action is being
held in abeyance, pending receipt of further information. The
SMTP client should send another command specifying this
information. This reply is used in command sequence groups (i.e.,
in DATA).
4yz Transient Negative Completion reply
The command was not accepted, and the requested action did not
occur. However, the error condition is temporary, and the action
may be requested again. The sender should return to the beginning
of the command sequence (if any). It is difficult to assign a
meaning to "transient" when two different sites (receiver- and
sender-SMTP agents) must agree on the interpretation. Each reply
in this category might have a different time value, but the SMTP
client SHOULD try again. A rule of thumb to determine whether a
reply fits into the 4yz or the 5yz category (see below) is that
replies are 4yz if they can be successful if repeated without any
change in command form or in properties of the sender or receiver
(that is, the command is repeated identically and the receiver
does not put up a new implementation).
5yz Permanent Negative Completion reply
The command was not accepted and the requested action did not
occur. The SMTP client SHOULD NOT repeat the exact request (in
the same sequence). Even some "permanent" error conditions can be
corrected, so the human user may want to direct the SMTP client to
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reinitiate the command sequence by direct action at some point in
the future (e.g., after the spelling has been changed, or the user
has altered the account status).
It is worth noting that the file transfer protocol (FTP) [34] uses a
very similar code architecture and that the SMTP codes are based on
the FTP model. However, SMTP uses a one-command, one-response model
(while FTP is asynchronous) and FTP's 1yz codes are not part of the
SMTP model.
The second digit encodes responses in specific categories:
x0z Syntax: These replies refer to syntax errors, syntactically
correct commands that do not fit any functional category, and
unimplemented or superfluous commands.
x1z Information: These are replies to requests for information, such
as status or help.
x2z Connections: These are replies referring to the transmission
channel.
x3z Unspecified.
x4z Unspecified.
x5z Mail system: These replies indicate the status of the receiver
mail system vis-a-vis the requested transfer or other mail system
action.
The third digit gives a finer gradation of meaning in each category
specified by the second digit. The list of replies illustrates this.
Each reply text is recommended rather than mandatory, and may even
change according to the command with which it is associated. On the
other hand, the reply codes must strictly follow the specifications
in this section. Receiver implementations should not invent new
codes for slightly different situations from the ones described here,
but rather adapt codes already defined.
For example, a command such as NOOP, whose successful execution does
not offer the SMTP client any new information, will return a 250
reply. The reply is 502 when the command requests an unimplemented
non-site-specific action. A refinement of that is the 504 reply for
a command that is implemented, but that requests an unimplemented
parameter.
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The reply text may be longer than a single line; in these cases the
complete text must be marked so the SMTP client knows when it can
stop reading the reply. This requires a special format to indicate a
multiple line reply.
The format for multiline replies requires that every line, except the
last, begin with the reply code, followed immediately by a hyphen,
"-" (also known as minus), followed by text. The last line will
begin with the reply code, followed immediately by <SP>, optionally
some text, and <CRLF>. As noted above, servers SHOULD send the <SP>
if subsequent text is not sent, but clients MUST be prepared for it
to be omitted.
For example:
250-First line
250-Second line
250-234 Text beginning with numbers
250 The last line
In a multiline reply, the reply code on each of the lines MUST be the
same. It is reasonable for the client to rely on this, so it can
make processing decisions based on the code in any line, assuming
that all others will be the same. In a few cases, there is important
data for the client in the reply "text". The client will be able to
identify these cases from the current context.
4.2.2. Reply Codes by Function Groups
500 Syntax error, command unrecognized (This may include errors such
as command line too long)
501 Syntax error in parameters or arguments
502 Command not implemented (see Section 4.2.4)
503 Bad sequence of commands
504 Command parameter not implemented
211 System status, or system help reply
214 Help message (Information on how to use the receiver or the
meaning of a particular non-standard command; this reply is useful
only to the human user)
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220 <domain> Service ready
221 <domain> Service closing transmission channel
421 <domain> Service not available, closing transmission channel
(This may be a reply to any command if the service knows it must
shut down)
250 Requested mail action okay, completed
251 User not local; will forward to <forward-path> (See Section 3.4)
252 Cannot VRFY user, but will accept message and attempt delivery
(See Section 3.5.3)
455 Server unable to accommodate parameters
555 MAIL FROM/RCPT TO parameters not recognized or not implemented
450 Requested mail action not taken: mailbox unavailable (e.g.,
mailbox busy or temporarily blocked for policy reasons)
550 Requested action not taken: mailbox unavailable (e.g., mailbox
not found, no access, or command rejected for policy reasons)
451 Requested action aborted: error in processing
551 User not local; please try <forward-path> (See Section 3.4)
452 Requested action not taken: insufficient system storage
552 Requested mail action aborted: exceeded storage allocation
553 Requested action not taken: mailbox name not allowed (e.g.,
mailbox syntax incorrect)
354 Start mail input; end with <CRLF>.<CRLF>
554 Transaction failed (Or, in the case of a connection-opening
response, "No SMTP service here")
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4.2.3. Reply Codes in Numeric Order
211 System status, or system help reply
214 Help message (Information on how to use the receiver or the
meaning of a particular non-standard command; this reply is useful
only to the human user)
220 <domain> Service ready
221 <domain> Service closing transmission channel
250 Requested mail action okay, completed
251 User not local; will forward to <forward-path> (See Section 3.4)
252 Cannot VRFY user, but will accept message and attempt delivery
(See Section 3.5.3)
354 Start mail input; end with <CRLF>.<CRLF>
421 <domain> Service not available, closing transmission channel
(This may be a reply to any command if the service knows it must
shut down)
450 Requested mail action not taken: mailbox unavailable (e.g.,
mailbox busy or temporarily blocked for policy reasons)
451 Requested action aborted: local error in processing
452 Requested action not taken: insufficient system storage
455 Server unable to accommodate parameters
500 Syntax error, command unrecognized (This may include errors such
as command line too long)
501 Syntax error in parameters or arguments
502 Command not implemented (see Section 4.2.4)
503 Bad sequence of commands
504 Command parameter not implemented
550 Requested action not taken: mailbox unavailable (e.g., mailbox
not found, no access, or command rejected for policy reasons)
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551 User not local; please try <forward-path> (See Section 3.4)
552 Requested mail action aborted: exceeded storage allocation
553 Requested action not taken: mailbox name not allowed (e.g.,
mailbox syntax incorrect)
554 Transaction failed (Or, in the case of a connection-opening
response, "No SMTP service here")
555 MAIL FROM/RCPT TO parameters not recognized or not implemented
4.2.4. Reply Code 502
Questions have been raised as to when reply code 502 (Command not
implemented) SHOULD be returned in preference to other codes. 502
SHOULD be used when the command is actually recognized by the SMTP
server, but not implemented. If the command is not recognized, code
500 SHOULD be returned. Extended SMTP systems MUST NOT list
capabilities in response to EHLO for which they will return 502 (or
500) replies.
4.2.5. Reply Codes after DATA and the Subsequent <CRLF>.<CRLF>
When an SMTP server returns a positive completion status (2yz code)
after the DATA command is completed with <CRLF>.<CRLF>, it accepts
responsibility for:
o delivering the message (if the recipient mailbox exists), or
o if attempts to deliver the message fail due to transient
conditions, retrying delivery some reasonable number of times at
intervals as specified in Section 4.5.4.
o if attempts to deliver the message fail due to permanent
conditions, or if repeated attempts to deliver the message fail
due to transient conditions, returning appropriate notification to
the sender of the original message (using the address in the SMTP
MAIL command).
When an SMTP server returns a temporary error status (4yz) code after
the DATA command is completed with <CRLF>.<CRLF>, it MUST NOT make a
subsequent attempt to deliver that message. The SMTP client retains
responsibility for the delivery of that message and may either return
it to the user or requeue it for a subsequent attempt (see
Section 4.5.4.1).
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The user who originated the message SHOULD be able to interpret the
return of a transient failure status (by mail message or otherwise)
as a non-delivery indication, just as a permanent failure would be
interpreted. If the client SMTP successfully handles these
conditions, the user will not receive such a reply.
When an SMTP server returns a permanent error status (5yz) code after
the DATA command is completed with <CRLF>.<CRLF>, it MUST NOT make
any subsequent attempt to deliver the message. As with temporary
error status codes, the SMTP client retains responsibility for the
message, but SHOULD not again attempt delivery to the same server
without user review of the message and response and appropriate
intervention.
4.3. Sequencing of Commands and Replies
4.3.1. Sequencing Overview
The communication between the sender and receiver is an alternating
dialogue, controlled by the sender. As such, the sender issues a
command and the receiver responds with a reply. Unless other
arrangements are negotiated through service extensions, the sender
MUST wait for this response before sending further commands. One
important reply is the connection greeting. Normally, a receiver
will send a 220 "Service ready" reply when the connection is
completed. The sender SHOULD wait for this greeting message before
sending any commands.
Note: all the greeting-type replies have the official name (the
fully-qualified primary domain name) of the server host as the first
word following the reply code. Sometimes the host will have no
meaningful name. See Section 4.1.3 for a discussion of alternatives
in these situations.
For example,
220 ISIF.USC.EDU Service ready
or
220 mail.example.com SuperSMTP v 6.1.2 Service ready
or
220 [10.0.0.1] Clueless host service ready
The table below lists alternative success and failure replies for
each command. These SHOULD be strictly adhered to. A receiver MAY
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substitute text in the replies, but the meanings and actions implied
by the code numbers and by the specific command reply sequence MUST
be preserved.
4.3.2. Command-Reply Sequences
Each command is listed with its usual possible replies. The prefixes
used before the possible replies are "I" for intermediate, "S" for
success, and "E" for error. Since some servers may generate other
replies under special circumstances, and to allow for future
extension, SMTP clients SHOULD, when possible, interpret only the
first digit of the reply and MUST be prepared to deal with
unrecognized reply codes by interpreting the first digit only.
Unless extended using the mechanisms described in Section 2.2, SMTP
servers MUST NOT transmit reply codes to an SMTP client that are
other than three digits or that do not start in a digit between 2 and
5 inclusive.
These sequencing rules and, in principle, the codes themselves, can
be extended or modified by SMTP extensions offered by the server and
accepted (requested) by the client. However, if the target is more
precise granularity in the codes, rather than codes for completely
new purposes, the system described in RFC 3463 [25] SHOULD be used in
preference to the invention of new codes.
In addition to the codes listed below, any SMTP command can return
any of the following codes if the corresponding unusual circumstances
are encountered:
500 For the "command line too long" case or if the command name was
not recognized. Note that producing a "command not recognized"
error in response to the required subset of these commands is a
violation of this specification. Similarly, producing a "command
too long" message for a command line shorter than 512 characters
would violate the provisions of Section 4.5.3.1.4.
501 Syntax error in command or arguments. In order to provide for
future extensions, commands that are specified in this document as
not accepting arguments (DATA, RSET, QUIT) SHOULD return a 501
message if arguments are supplied in the absence of EHLO-
advertised extensions.
421 Service shutting down and closing transmission channel
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Specific sequences are:
CONNECTION ESTABLISHMENT
S: 220
E: 554
EHLO or HELO
S: 250
E: 504 (a conforming implementation could return this code only
in fairly obscure cases), 550, 502 (permitted only with an old-
style server that does not support EHLO)
MAIL
S: 250
E: 552, 451, 452, 550, 553, 503, 455, 555
RCPT
S: 250, 251 (but see Section 3.4 for discussion of 251 and 551)
E: 550, 551, 552, 553, 450, 451, 452, 503, 455, 555
DATA
I: 354 -> data -> S: 250
E: 552, 554, 451, 452
E: 450, 550 (rejections for policy reasons)
E: 503, 554
RSET
S: 250
VRFY
S: 250, 251, 252
E: 550, 551, 553, 502, 504
EXPN
S: 250, 252
E: 550, 500, 502, 504
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HELP
S: 211, 214
E: 502, 504
NOOP
S: 250
QUIT
S: 221
4.4. Trace Information
When an SMTP server receives a message for delivery or further
processing, it MUST insert trace ("time stamp" or "Received")
information at the beginning of the message content, as discussed in
Section 4.1.1.4.
This line MUST be structured as follows:
o The FROM clause, which MUST be supplied in an SMTP environment,
SHOULD contain both (1) the name of the source host as presented
in the EHLO command and (2) an address literal containing the IP
address of the source, determined from the TCP connection.
o The ID clause MAY contain an "@" as suggested in RFC 822, but this
is not required.
o If the FOR clause appears, it MUST contain exactly one <path>
entry, even when multiple RCPT commands have been given. Multiple
<path>s raise some security issues and have been deprecated, see
Section 7.2.
An Internet mail program MUST NOT change or delete a Received: line
that was previously added to the message header section. SMTP
servers MUST prepend Received lines to messages; they MUST NOT change
the order of existing lines or insert Received lines in any other
location.
As the Internet grows, comparability of Received header fields is
important for detecting problems, especially slow relays. SMTP
servers that create Received header fields SHOULD use explicit
offsets in the dates (e.g., -0800), rather than time zone names of
any type. Local time (with an offset) SHOULD be used rather than UT
when feasible. This formulation allows slightly more information
about local circumstances to be specified. If UT is needed, the
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receiver need merely do some simple arithmetic to convert the values.
Use of UT loses information about the time zone-location of the
server. If it is desired to supply a time zone name, it SHOULD be
included in a comment.
When the delivery SMTP server makes the "final delivery" of a
message, it inserts a return-path line at the beginning of the mail
data. This use of return-path is required; mail systems MUST support
it. The return-path line preserves the information in the <reverse-
path> from the MAIL command. Here, final delivery means the message
has left the SMTP environment. Normally, this would mean it had been
delivered to the destination user or an associated mail drop, but in
some cases it may be further processed and transmitted by another
mail system.
It is possible for the mailbox in the return path to be different
from the actual sender's mailbox, for example, if error responses are
to be delivered to a special error handling mailbox rather than to
the message sender. When mailing lists are involved, this
arrangement is common and useful as a means of directing errors to
the list maintainer rather than the message originator.
The text above implies that the final mail data will begin with a
return path line, followed by one or more time stamp lines. These
lines will be followed by the rest of the mail data: first the
balance of the mail header section and then the body (RFC 5322 [4]).
It is sometimes difficult for an SMTP server to determine whether or
not it is making final delivery since forwarding or other operations
may occur after the message is accepted for delivery. Consequently,
any further (forwarding, gateway, or relay) systems MAY remove the
return path and rebuild the MAIL command as needed to ensure that
exactly one such line appears in a delivered message.
A message-originating SMTP system SHOULD NOT send a message that
already contains a Return-path header field. SMTP servers performing
a relay function MUST NOT inspect the message data, and especially
not to the extent needed to determine if Return-path header fields
are present. SMTP servers making final delivery MAY remove Return-
path header fields before adding their own.
The primary purpose of the Return-path is to designate the address to
which messages indicating non-delivery or other mail system failures
are to be sent. For this to be unambiguous, exactly one return path
SHOULD be present when the message is delivered. Systems using RFC
822 syntax with non-SMTP transports SHOULD designate an unambiguous
address, associated with the transport envelope, to which error
reports (e.g., non-delivery messages) should be sent.
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Historical note: Text in RFC 822 that appears to contradict the use
of the Return-path header field (or the envelope reverse-path address
from the MAIL command) as the destination for error messages is not
applicable on the Internet. The reverse-path address (as copied into
the Return-path) MUST be used as the target of any mail containing
delivery error messages.
In particular:
o a gateway from SMTP -> elsewhere SHOULD insert a return-path
header field, unless it is known that the "elsewhere" transport
also uses Internet domain addresses and maintains the envelope
sender address separately.
o a gateway from elsewhere -> SMTP SHOULD delete any return-path
header field present in the message, and either copy that
information to the SMTP envelope or combine it with information
present in the envelope of the other transport system to construct
the reverse-path argument to the MAIL command in the SMTP
envelope.
The server must give special treatment to cases in which the
processing following the end of mail data indication is only
partially successful. This could happen if, after accepting several
recipients and the mail data, the SMTP server finds that the mail
data could be successfully delivered to some, but not all, of the
recipients. In such cases, the response to the DATA command MUST be
an OK reply. However, the SMTP server MUST compose and send an
"undeliverable mail" notification message to the originator of the
message.
A single notification listing all of the failed recipients or
separate notification messages MUST be sent for each failed
recipient. For economy of processing by the sender, the former
SHOULD be used when possible. Note that the key difference between
handling aliases (Section 3.9.1) and forwarding (this subsection) is
the change to the backward-pointing address in this case. All
notification messages about undeliverable mail MUST be sent using the
MAIL command (even if they result from processing the obsolete SEND,
SOML, or SAML commands) and MUST use a null return path as discussed
in Section 3.6.
The time stamp line and the return path line are formally defined as
follows (the definitions for "FWS" and "CFWS" appear in RFC 5322
[4]):
Return-path-line = "Return-Path:" FWS Reverse-path <CRLF>
Time-stamp-line = "Received:" FWS Stamp <CRLF>
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Stamp = From-domain By-domain Opt-info [CFWS] ";"
FWS date-time
; where "date-time" is as defined in RFC 5322 [4]
; but the "obs-" forms, especially two-digit
; years, are prohibited in SMTP and MUST NOT be used.
From-domain = "FROM" FWS Extended-Domain
By-domain = CFWS "BY" FWS Extended-Domain
Extended-Domain = Domain /
( Domain FWS "(" TCP-info ")" ) /
( address-literal FWS "(" TCP-info ")" )
TCP-info = address-literal / ( Domain FWS address-literal )
; Information derived by server from TCP connection
; not client EHLO.
Opt-info = [Via] [With] [ID] [For]
[Additional-Registered-Clauses]
Via = CFWS "VIA" FWS Link
With = CFWS "WITH" FWS Protocol
ID = CFWS "ID" FWS ( Atom / msg-id )
; msg-id is defined in RFC 5322 [4]
For = CFWS "FOR" FWS ( Path / Mailbox )
Additional-Registered-Clauses = CFWS Atom FWS String
; Additional standard clauses may be
added in this
; location by future standards and
registration with
; IANA. SMTP servers SHOULD NOT use
unregistered
; names. See Section 8.
Link = "TCP" / Addtl-Link
Addtl-Link = Atom
; Additional standard names for links are
; registered with the Internet Assigned Numbers
; Authority (IANA). "Via" is primarily of value
; with non-Internet transports. SMTP servers
; SHOULD NOT use unregistered names.
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Protocol = "ESMTP" / "SMTP" / Attdl-Protocol
Attdl-Protocol = Atom
; Additional standard names for protocols are
; registered with the Internet Assigned Numbers
; Authority (IANA) in the "mail parameters"
; registry [9]. SMTP servers SHOULD NOT
; use unregistered names.
4.5. Additional Implementation Issues
4.5.1. Minimum Implementation
In order to make SMTP workable, the following minimum implementation
MUST be provided by all receivers. The following commands MUST be
supported to conform to this specification:
EHLO
HELO
MAIL
RCPT
DATA
RSET
NOOP
QUIT
VRFY
Any system that includes an SMTP server supporting mail relaying or
delivery MUST support the reserved mailbox "postmaster" as a case-
insensitive local name. This postmaster address is not strictly
necessary if the server always returns 554 on connection opening (as
described in Section 3.1). The requirement to accept mail for
postmaster implies that RCPT commands that specify a mailbox for
postmaster at any of the domains for which the SMTP server provides
mail service, as well as the special case of "RCPT TO:<Postmaster>"
(with no domain specification), MUST be supported.
SMTP systems are expected to make every reasonable effort to accept
mail directed to Postmaster from any other system on the Internet.
In extreme cases -- such as to contain a denial of service attack or
other breach of security -- an SMTP server may block mail directed to
Postmaster. However, such arrangements SHOULD be narrowly tailored
so as to avoid blocking messages that are not part of such attacks.
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4.5.2. Transparency
Without some provision for data transparency, the character sequence
"<CRLF>.<CRLF>" ends the mail text and cannot be sent by the user.
In general, users are not aware of such "forbidden" sequences. To
allow all user composed text to be transmitted transparently, the
following procedures are used:
o Before sending a line of mail text, the SMTP client checks the
first character of the line. If it is a period, one additional
period is inserted at the beginning of the line.
o When a line of mail text is received by the SMTP server, it checks
the line. If the line is composed of a single period, it is
treated as the end of mail indicator. If the first character is a
period and there are other characters on the line, the first
character is deleted.
The mail data may contain any of the 128 ASCII characters. All
characters are to be delivered to the recipient's mailbox, including
spaces, vertical and horizontal tabs, and other control characters.
If the transmission channel provides an 8-bit byte (octet) data
stream, the 7-bit ASCII codes are transmitted, right justified, in
the octets, with the high-order bits cleared to zero. See
Section 3.6 for special treatment of these conditions in SMTP systems
serving a relay function.
In some systems, it may be necessary to transform the data as it is
received and stored. This may be necessary for hosts that use a
different character set than ASCII as their local character set, that
store data in records rather than strings, or which use special
character sequences as delimiters inside mailboxes. If such
transformations are necessary, they MUST be reversible, especially if
they are applied to mail being relayed.
4.5.3. Sizes and Timeouts
4.5.3.1. Size Limits and Minimums
There are several objects that have required minimum/maximum sizes.
Every implementation MUST be able to receive objects of at least
these sizes. Objects larger than these sizes SHOULD be avoided when
possible. However, some Internet mail constructs such as encoded
X.400 addresses (RFC 2156 [35]) will often require larger objects.
Clients MAY attempt to transmit these, but MUST be prepared for a
server to reject them if they cannot be handled by it. To the
maximum extent possible, implementation techniques that impose no
limits on the length of these objects should be used.
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Extensions to SMTP may involve the use of characters that occupy more
than a single octet each. This section therefore specifies lengths
in octets where absolute lengths, rather than character counts, are
intended.
4.5.3.1.1. Local-part
The maximum total length of a user name or other local-part is 64
octets.
4.5.3.1.2. Domain
The maximum total length of a domain name or number is 255 octets.
4.5.3.1.3. Path
The maximum total length of a reverse-path or forward-path is 256
octets (including the punctuation and element separators).
4.5.3.1.4. Command Line
The maximum total length of a command line including the command word
and the <CRLF> is 512 octets. SMTP extensions may be used to
increase this limit.
4.5.3.1.5. Reply Line
The maximum total length of a reply line including the reply code and
the <CRLF> is 512 octets. More information may be conveyed through
multiple-line replies.
4.5.3.1.6. Text Line
The maximum total length of a text line including the <CRLF> is 1000
octets (not counting the leading dot duplicated for transparency).
This number may be increased by the use of SMTP Service Extensions.
4.5.3.1.7. Message Content
The maximum total length of a message content (including any message
header section as well as the message body) MUST BE at least 64K
octets. Since the introduction of Internet Standards for multimedia
mail (RFC 2045 [21]), message lengths on the Internet have grown
dramatically, and message size restrictions should be avoided if at
all possible. SMTP server systems that must impose restrictions
SHOULD implement the "SIZE" service extension of RFC 1870 [10], and
SMTP client systems that will send large messages SHOULD utilize it
when possible.
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4.5.3.1.8. Recipients Buffer
The minimum total number of recipients that MUST be buffered is 100
recipients. Rejection of messages (for excessive recipients) with
fewer than 100 RCPT commands is a violation of this specification.
The general principle that relaying SMTP server MUST NOT, and
delivery SMTP servers SHOULD NOT, perform validation tests on message
header fields suggests that messages SHOULD NOT be rejected based on
the total number of recipients shown in header fields. A server that
imposes a limit on the number of recipients MUST behave in an orderly
fashion, such as rejecting additional addresses over its limit rather
than silently discarding addresses previously accepted. A client
that needs to deliver a message containing over 100 RCPT commands
SHOULD be prepared to transmit in 100-recipient "chunks" if the
server declines to accept more than 100 recipients in a single
message.
4.5.3.1.9. Treatment When Limits Exceeded
Errors due to exceeding these limits may be reported by using the
reply codes. Some examples of reply codes are:
500 Line too long.
or
501 Path too long
or
452 Too many recipients (see below)
or
552 Too much mail data.
4.5.3.1.10. Too Many Recipients Code
RFC 821 [1] incorrectly listed the error where an SMTP server
exhausts its implementation limit on the number of RCPT commands
("too many recipients") as having reply code 552. The correct reply
code for this condition is 452. Clients SHOULD treat a 552 code in
this case as a temporary, rather than permanent, failure so the logic
below works.
When a conforming SMTP server encounters this condition, it has at
least 100 successful RCPT commands in its recipients buffer. If the
server is able to accept the message, then at least these 100
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addresses will be removed from the SMTP client's queue. When the
client attempts retransmission of those addresses that received 452
responses, at least 100 of these will be able to fit in the SMTP
server's recipients buffer. Each retransmission attempt that is able
to deliver anything will be able to dispose of at least 100 of these
recipients.
If an SMTP server has an implementation limit on the number of RCPT
commands and this limit is exhausted, it MUST use a response code of
452 (but the client SHOULD also be prepared for a 552, as noted
above). If the server has a configured site-policy limitation on the
number of RCPT commands, it MAY instead use a 5yz response code. In
particular, if the intent is to prohibit messages with more than a
site-specified number of recipients, rather than merely limit the
number of recipients in a given mail transaction, it would be
reasonable to return a 503 response to any DATA command received
subsequent to the 452 (or 552) code or to simply return the 503 after
DATA without returning any previous negative response.
4.5.3.2. Timeouts
An SMTP client MUST provide a timeout mechanism. It MUST use per-
command timeouts rather than somehow trying to time the entire mail
transaction. Timeouts SHOULD be easily reconfigurable, preferably
without recompiling the SMTP code. To implement this, a timer is set
for each SMTP command and for each buffer of the data transfer. The
latter means that the overall timeout is inherently proportional to
the size of the message.
Based on extensive experience with busy mail-relay hosts, the minimum
per-command timeout values SHOULD be as follows:
4.5.3.2.1. Initial 220 Message: 5 Minutes
An SMTP client process needs to distinguish between a failed TCP
connection and a delay in receiving the initial 220 greeting message.
Many SMTP servers accept a TCP connection but delay delivery of the
220 message until their system load permits more mail to be
processed.
4.5.3.2.2. MAIL Command: 5 Minutes
4.5.3.2.3. RCPT Command: 5 Minutes
A longer timeout is required if processing of mailing lists and
aliases is not deferred until after the message was accepted.
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4.5.3.2.4. DATA Initiation: 2 Minutes
This is while awaiting the "354 Start Input" reply to a DATA command.
4.5.3.2.5. Data Block: 3 Minutes
This is while awaiting the completion of each TCP SEND call
transmitting a chunk of data.
4.5.3.2.6. DATA Termination: 10 Minutes.
This is while awaiting the "250 OK" reply. When the receiver gets
the final period terminating the message data, it typically performs
processing to deliver the message to a user mailbox. A spurious
timeout at this point would be very wasteful and would typically
result in delivery of multiple copies of the message, since it has
been successfully sent and the server has accepted responsibility for
delivery. See Section 6.1 for additional discussion.
4.5.3.2.7. Server Timeout: 5 Minutes.
An SMTP server SHOULD have a timeout of at least 5 minutes while it
is awaiting the next command from the sender.
4.5.4. Retry Strategies
The common structure of a host SMTP implementation includes user
mailboxes, one or more areas for queuing messages in transit, and one
or more daemon processes for sending and receiving mail. The exact
structure will vary depending on the needs of the users on the host
and the number and size of mailing lists supported by the host. We
describe several optimizations that have proved helpful, particularly
for mailers supporting high traffic levels.
Any queuing strategy MUST include timeouts on all activities on a
per-command basis. A queuing strategy MUST NOT send error messages
in response to error messages under any circumstances.
4.5.4.1. Sending Strategy
The general model for an SMTP client is one or more processes that
periodically attempt to transmit outgoing mail. In a typical system,
the program that composes a message has some method for requesting
immediate attention for a new piece of outgoing mail, while mail that
cannot be transmitted immediately MUST be queued and periodically
retried by the sender. A mail queue entry will include not only the
message itself but also the envelope information.
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The sender MUST delay retrying a particular destination after one
attempt has failed. In general, the retry interval SHOULD be at
least 30 minutes; however, more sophisticated and variable strategies
will be beneficial when the SMTP client can determine the reason for
non-delivery.
Retries continue until the message is transmitted or the sender gives
up; the give-up time generally needs to be at least 4-5 days. It MAY
be appropriate to set a shorter maximum number of retries for non-
delivery notifications and equivalent error messages than for
standard messages. The parameters to the retry algorithm MUST be
configurable.
A client SHOULD keep a list of hosts it cannot reach and
corresponding connection timeouts, rather than just retrying queued
mail items.
Experience suggests that failures are typically transient (the target
system or its connection has crashed), favoring a policy of two
connection attempts in the first hour the message is in the queue,
and then backing off to one every two or three hours.
The SMTP client can shorten the queuing delay in cooperation with the
SMTP server. For example, if mail is received from a particular
address, it is likely that mail queued for that host can now be sent.
Application of this principle may, in many cases, eliminate the
requirement for an explicit "send queues now" function such as ETRN,
RFC 1985 [36].
The strategy may be further modified as a result of multiple
addresses per host (see below) to optimize delivery time versus
resource usage.
An SMTP client may have a large queue of messages for each
unavailable destination host. If all of these messages were retried
in every retry cycle, there would be excessive Internet overhead and
the sending system would be blocked for a long period. Note that an
SMTP client can generally determine that a delivery attempt has
failed only after a timeout of several minutes, and even a one-minute
timeout per connection will result in a very large delay if retries
are repeated for dozens, or even hundreds, of queued messages to the
same host.
At the same time, SMTP clients SHOULD use great care in caching
negative responses from servers. In an extreme case, if EHLO is
issued multiple times during the same SMTP connection, different
answers may be returned by the server. More significantly, 5yz
responses to the MAIL command MUST NOT be cached.
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When a mail message is to be delivered to multiple recipients, and
the SMTP server to which a copy of the message is to be sent is the
same for multiple recipients, then only one copy of the message
SHOULD be transmitted. That is, the SMTP client SHOULD use the
command sequence: MAIL, RCPT, RCPT, ..., RCPT, DATA instead of the
sequence: MAIL, RCPT, DATA, ..., MAIL, RCPT, DATA. However, if there
are very many addresses, a limit on the number of RCPT commands per
MAIL command MAY be imposed. This efficiency feature SHOULD be
implemented.
Similarly, to achieve timely delivery, the SMTP client MAY support
multiple concurrent outgoing mail transactions. However, some limit
may be appropriate to protect the host from devoting all its
resources to mail.
4.5.4.2. Receiving Strategy
The SMTP server SHOULD attempt to keep a pending listen on the SMTP
port (specified by IANA as port 25) at all times. This requires the
support of multiple incoming TCP connections for SMTP. Some limit
MAY be imposed, but servers that cannot handle more than one SMTP
transaction at a time are not in conformance with the intent of this
specification.
As discussed above, when the SMTP server receives mail from a
particular host address, it could activate its own SMTP queuing
mechanisms to retry any mail pending for that host address.
4.5.5. Messages with a Null Reverse-Path
There are several types of notification messages that are required by
existing and proposed Standards to be sent with a null reverse-path,
namely non-delivery notifications as discussed in Section 3.7, other
kinds of Delivery Status Notifications (DSNs, RFC 3461 [32]), and
Message Disposition Notifications (MDNs, RFC 3798 [37]). All of
these kinds of messages are notifications about a previous message,
and they are sent to the reverse-path of the previous mail message.
(If the delivery of such a notification message fails, that usually
indicates a problem with the mail system of the host to which the
notification message is addressed. For this reason, at some hosts
the MTA is set up to forward such failed notification messages to
someone who is able to fix problems with the mail system, e.g., via
the postmaster alias.)
All other types of messages (i.e., any message which is not required
by a Standards-Track RFC to have a null reverse-path) SHOULD be sent
with a valid, non-null reverse-path.
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Implementers of automated email processors should be careful to make
sure that the various kinds of messages with a null reverse-path are
handled correctly. In particular, such systems SHOULD NOT reply to
messages with a null reverse-path, and they SHOULD NOT add a non-null
reverse-path, or change a null reverse-path to a non-null one, to
such messages when forwarding.
5. Address Resolution and Mail Handling
5.1. Locating the Target Host
Once an SMTP client lexically identifies a domain to which mail will
be delivered for processing (as described in Sections 2.3.5 and 3.6),
a DNS lookup MUST be performed to resolve the domain name (RFC 1035
[2]). The names are expected to be fully-qualified domain names
(FQDNs): mechanisms for inferring FQDNs from partial names or local
aliases are outside of this specification. Due to a history of
problems, SMTP servers used for initial submission of messages SHOULD
NOT make such inferences (Message Submission Servers [18] have
somewhat more flexibility) and intermediate (relay) SMTP servers MUST
NOT make them.
The lookup first attempts to locate an MX record associated with the
name. If a CNAME record is found, the resulting name is processed as
if it were the initial name. If a non-existent domain error is
returned, this situation MUST be reported as an error. If a
temporary error is returned, the message MUST be queued and retried
later (see Section 4.5.4.1). If an empty list of MXs is returned,
the address is treated as if it was associated with an implicit MX
RR, with a preference of 0, pointing to that host. If MX records are
present, but none of them are usable, or the implicit MX is unusable,
this situation MUST be reported as an error.
If one or more MX RRs are found for a given name, SMTP systems MUST
NOT utilize any address RRs associated with that name unless they are
located using the MX RRs; the "implicit MX" rule above applies only
if there are no MX records present. If MX records are present, but
none of them are usable, this situation MUST be reported as an error.
When a domain name associated with an MX RR is looked up and the
associated data field obtained, the data field of that response MUST
contain a domain name. That domain name, when queried, MUST return
at least one address record (e.g., A or AAAA RR) that gives the IP
address of the SMTP server to which the message should be directed.
Any other response, specifically including a value that will return a
CNAME record when queried, lies outside the scope of this Standard.
The prohibition on labels in the data that resolve to CNAMEs is
discussed in more detail in RFC 2181, Section 10.3 [38].
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When the lookup succeeds, the mapping can result in a list of
alternative delivery addresses rather than a single address, because
of multiple MX records, multihoming, or both. To provide reliable
mail transmission, the SMTP client MUST be able to try (and retry)
each of the relevant addresses in this list in order, until a
delivery attempt succeeds. However, there MAY also be a configurable
limit on the number of alternate addresses that can be tried. In any
case, the SMTP client SHOULD try at least two addresses.
Two types of information are used to rank the host addresses:
multiple MX records, and multihomed hosts.
MX records contain a preference indication that MUST be used in
sorting if more than one such record appears (see below). Lower
numbers are more preferred than higher ones. If there are multiple
destinations with the same preference and there is no clear reason to
favor one (e.g., by recognition of an easily reached address), then
the sender-SMTP MUST randomize them to spread the load across
multiple mail exchangers for a specific organization.
The destination host (perhaps taken from the preferred MX record) may
be multihomed, in which case the domain name resolver will return a
list of alternative IP addresses. It is the responsibility of the
domain name resolver interface to have ordered this list by
decreasing preference if necessary, and the SMTP sender MUST try them
in the order presented.
Although the capability to try multiple alternative addresses is
required, specific installations may want to limit or disable the use
of alternative addresses. The question of whether a sender should
attempt retries using the different addresses of a multihomed host
has been controversial. The main argument for using the multiple
addresses is that it maximizes the probability of timely delivery,
and indeed sometimes the probability of any delivery; the counter-
argument is that it may result in unnecessary resource use. Note
that resource use is also strongly determined by the sending strategy
discussed in Section 4.5.4.1.
If an SMTP server receives a message with a destination for which it
is a designated Mail eXchanger, it MAY relay the message (potentially
after having rewritten the MAIL FROM and/or RCPT TO addresses), make
final delivery of the message, or hand it off using some mechanism
outside the SMTP-provided transport environment. Of course, neither
of the latter require that the list of MX records be examined
further.
If it determines that it should relay the message without rewriting
the address, it MUST sort the MX records to determine candidates for
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delivery. The records are first ordered by preference, with the
lowest-numbered records being most preferred. The relay host MUST
then inspect the list for any of the names or addresses by which it
might be known in mail transactions. If a matching record is found,
all records at that preference level and higher-numbered ones MUST be
discarded from consideration. If there are no records left at that
point, it is an error condition, and the message MUST be returned as
undeliverable. If records do remain, they SHOULD be tried, best
preference first, as described above.
5.2. IPv6 and MX Records
In the contemporary Internet, SMTP clients and servers may be hosted
on IPv4 systems, IPv6 systems, or dual-stack systems that are
compatible with either version of the Internet Protocol. The host
domains to which MX records point may, consequently, contain "A RR"s
(IPv4), "AAAA RR"s (IPv6), or any combination of them. While RFC
3974 [39] discusses some operational experience in mixed
environments, it was not comprehensive enough to justify
standardization, and some of its recommendations appear to be
inconsistent with this specification. The appropriate actions to be
taken either will depend on local circumstances, such as performance
of the relevant networks and any conversions that might be necessary,
or will be obvious (e.g., an IPv6-only client need not attempt to
look up A RRs or attempt to reach IPv4-only servers). Designers of
SMTP implementations that might run in IPv6 or dual-stack
environments should study the procedures above, especially the
comments about multihomed hosts, and, preferably, provide mechanisms
to facilitate operational tuning and mail interoperability between
IPv4 and IPv6 systems while considering local circumstances.
6. Problem Detection and Handling
6.1. Reliable Delivery and Replies by Email
When the receiver-SMTP accepts a piece of mail (by sending a "250 OK"
message in response to DATA), it is accepting responsibility for
delivering or relaying the message. It must take this responsibility
seriously. It MUST NOT lose the message for frivolous reasons, such
as because the host later crashes or because of a predictable
resource shortage. Some reasons that are not considered frivolous
are discussed in the next subsection and in Section 7.8.
If there is a delivery failure after acceptance of a message, the
receiver-SMTP MUST formulate and mail a notification message. This
notification MUST be sent using a null ("<>") reverse-path in the
envelope. The recipient of this notification MUST be the address
from the envelope return path (or the Return-Path: line). However,
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if this address is null ("<>"), the receiver-SMTP MUST NOT send a
notification. Obviously, nothing in this section can or should
prohibit local decisions (i.e., as part of the same system
environment as the receiver-SMTP) to log or otherwise transmit
information about null address events locally if that is desired. If
the address is an explicit source route, it MUST be stripped down to
its final hop.
For example, suppose that an error notification must be sent for a
message that arrived with:
MAIL FROM:<@a,@b:user@d>
The notification message MUST be sent using:
RCPT TO:<user@d>
Some delivery failures after the message is accepted by SMTP will be
unavoidable. For example, it may be impossible for the receiving
SMTP server to validate all the delivery addresses in RCPT command(s)
due to a "soft" domain system error, because the target is a mailing
list (see earlier discussion of RCPT), or because the server is
acting as a relay and has no immediate access to the delivering
system.
To avoid receiving duplicate messages as the result of timeouts, a
receiver-SMTP MUST seek to minimize the time required to respond to
the final <CRLF>.<CRLF> end of data indicator. See RFC 1047 [40] for
a discussion of this problem.
6.2. Unwanted, Unsolicited, and "Attack" Messages
Utility and predictability of the Internet mail system requires that
messages that can be delivered should be delivered, regardless of any
syntax or other faults associated with those messages and regardless
of their content. If they cannot be delivered, and cannot be
rejected by the SMTP server during the SMTP transaction, they should
be "bounced" (returned with non-delivery notification messages) as
described above. In today's world, in which many SMTP server
operators have discovered that the quantity of undesirable bulk email
vastly exceeds the quantity of desired mail and in which accepting a
message may trigger additional undesirable traffic by providing
verification of the address, those principles may not be practical.
As discussed in Section 7.8 and Section 7.9 below, dropping mail
without notification of the sender is permitted in practice.
However, it is extremely dangerous and violates a long tradition and
community expectations that mail is either delivered or returned. If
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silent message-dropping is misused, it could easily undermine
confidence in the reliability of the Internet's mail systems. So
silent dropping of messages should be considered only in those cases
where there is very high confidence that the messages are seriously
fraudulent or otherwise inappropriate.
To stretch the principle of delivery if possible even further, it may
be a rational policy to not deliver mail that has an invalid return
address, although the history of the network is that users are
typically better served by delivering any message that can be
delivered. Reliably determining that a return address is invalid can
be a difficult and time-consuming process, especially if the putative
sending system is not directly accessible or does not fully and
accurately support VRFY and, even if a "drop messages with invalid
return addresses" policy is adopted, it SHOULD be applied only when
there is near-certainty that the return addresses are, in fact,
invalid.
Conversely, if a message is rejected because it is found to contain
hostile content (a decision that is outside the scope of an SMTP
server as defined in this document), rejection ("bounce") messages
SHOULD NOT be sent unless the receiving site is confident that those
messages will be usefully delivered. The preference and default in
these cases is to avoid sending non-delivery messages when the
incoming message is determined to contain hostile content.
6.3. Loop Detection
Simple counting of the number of "Received:" header fields in a
message has proven to be an effective, although rarely optimal,
method of detecting loops in mail systems. SMTP servers using this
technique SHOULD use a large rejection threshold, normally at least
100 Received entries. Whatever mechanisms are used, servers MUST
contain provisions for detecting and stopping trivial loops.
6.4. Compensating for Irregularities
Unfortunately, variations, creative interpretations, and outright
violations of Internet mail protocols do occur; some would suggest
that they occur quite frequently. The debate as to whether a well-
behaved SMTP receiver or relay should reject a malformed message,
attempt to pass it on unchanged, or attempt to repair it to increase
the odds of successful delivery (or subsequent reply) began almost
with the dawn of structured network mail and shows no signs of
abating. Advocates of rejection claim that attempted repairs are
rarely completely adequate and that rejection of bad messages is the
only way to get the offending software repaired. Advocates of
"repair" or "deliver no matter what" argue that users prefer that
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mail go through it if at all possible and that there are significant
market pressures in that direction. In practice, these market
pressures may be more important to particular vendors than strict
conformance to the standards, regardless of the preference of the
actual developers.
The problems associated with ill-formed messages were exacerbated by
the introduction of the split-UA mail reading protocols (Post Office
Protocol (POP) version 2 [15], Post Office Protocol (POP) version 3
[16], IMAP version 2 [41], and PCMAIL [42]). These protocols
encouraged the use of SMTP as a posting (message submission)
protocol, and SMTP servers as relay systems for these client hosts
(which are often only intermittently connected to the Internet).
Historically, many of those client machines lacked some of the
mechanisms and information assumed by SMTP (and indeed, by the mail
format protocol, RFC 822 [28]). Some could not keep adequate track
of time; others had no concept of time zones; still others could not
identify their own names or addresses; and, of course, none could
satisfy the assumptions that underlay RFC 822's conception of
authenticated addresses.
In response to these weak SMTP clients, many SMTP systems now
complete messages that are delivered to them in incomplete or
incorrect form. This strategy is generally considered appropriate
when the server can identify or authenticate the client, and there
are prior agreements between them. By contrast, there is at best
great concern about fixes applied by a relay or delivery SMTP server
that has little or no knowledge of the user or client machine. Many
of these issues are addressed by using a separate protocol, such as
that defined in RFC 4409 [18], for message submission, rather than
using originating SMTP servers for that purpose.
The following changes to a message being processed MAY be applied
when necessary by an originating SMTP server, or one used as the
target of SMTP as an initial posting (message submission) protocol:
o Addition of a message-id field when none appears
o Addition of a date, time, or time zone when none appears
o Correction of addresses to proper FQDN format
The less information the server has about the client, the less likely
these changes are to be correct and the more caution and conservatism
should be applied when considering whether or not to perform fixes
and how. These changes MUST NOT be applied by an SMTP server that
provides an intermediate relay function.
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In all cases, properly operating clients supplying correct
information are preferred to corrections by the SMTP server. In all
cases, documentation SHOULD be provided in trace header fields and/or
header field comments for actions performed by the servers.
7. Security Considerations
7.1. Mail Security and Spoofing
SMTP mail is inherently insecure in that it is feasible for even
fairly casual users to negotiate directly with receiving and relaying
SMTP servers and create messages that will trick a naive recipient
into believing that they came from somewhere else. Constructing such
a message so that the "spoofed" behavior cannot be detected by an
expert is somewhat more difficult, but not sufficiently so as to be a
deterrent to someone who is determined and knowledgeable.
Consequently, as knowledge of Internet mail increases, so does the
knowledge that SMTP mail inherently cannot be authenticated, or
integrity checks provided, at the transport level. Real mail
security lies only in end-to-end methods involving the message
bodies, such as those that use digital signatures (see RFC 1847 [43]
and, e.g., Pretty Good Privacy (PGP) in RFC 4880 [44] or Secure/
Multipurpose Internet Mail Extensions (S/MIME) in RFC 3851 [45]).
Various protocol extensions and configuration options that provide
authentication at the transport level (e.g., from an SMTP client to
an SMTP server) improve somewhat on the traditional situation
described above. However, in general, they only authenticate one
server to another rather than a chain of relays and servers, much
less authenticating users or user machines. Consequently, unless
they are accompanied by careful handoffs of responsibility in a
carefully designed trust environment, they remain inherently weaker
than end-to-end mechanisms that use digitally signed messages rather
than depending on the integrity of the transport system.
Efforts to make it more difficult for users to set envelope return
path and header "From" fields to point to valid addresses other than
their own are largely misguided: they frustrate legitimate
applications in which mail is sent by one user on behalf of another,
in which error (or normal) replies should be directed to a special
address, or in which a single message is sent to multiple recipients
on different hosts. (Systems that provide convenient ways for users
to alter these header fields on a per-message basis should attempt to
establish a primary and permanent mailbox address for the user so
that Sender header fields within the message data can be generated
sensibly.)
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This specification does not further address the authentication issues
associated with SMTP other than to advocate that useful functionality
not be disabled in the hope of providing some small margin of
protection against a user who is trying to fake mail.
7.2. "Blind" Copies
Addresses that do not appear in the message header section may appear
in the RCPT commands to an SMTP server for a number of reasons. The
two most common involve the use of a mailing address as a "list
exploder" (a single address that resolves into multiple addresses)
and the appearance of "blind copies". Especially when more than one
RCPT command is present, and in order to avoid defeating some of the
purpose of these mechanisms, SMTP clients and servers SHOULD NOT copy
the full set of RCPT command arguments into the header section,
either as part of trace header fields or as informational or private-
extension header fields. Since this rule is often violated in
practice, and cannot be enforced, sending SMTP systems that are aware
of "bcc" use MAY find it helpful to send each blind copy as a
separate message transaction containing only a single RCPT command.
There is no inherent relationship between either "reverse" (from
MAIL, SAML, etc., commands) or "forward" (RCPT) addresses in the SMTP
transaction ("envelope") and the addresses in the header section.
Receiving systems SHOULD NOT attempt to deduce such relationships and
use them to alter the header section of the message for delivery.
The popular "Apparently-to" header field is a violation of this
principle as well as a common source of unintended information
disclosure and SHOULD NOT be used.
7.3. VRFY, EXPN, and Security
As discussed in Section 3.5, individual sites may want to disable
either or both of VRFY or EXPN for security reasons (see below). As
a corollary to the above, implementations that permit this MUST NOT
appear to have verified addresses that are not, in fact, verified.
If a site disables these commands for security reasons, the SMTP
server MUST return a 252 response, rather than a code that could be
confused with successful or unsuccessful verification.
Returning a 250 reply code with the address listed in the VRFY
command after having checked it only for syntax violates this rule.
Of course, an implementation that "supports" VRFY by always returning
550 whether or not the address is valid is equally not in
conformance.
On the public Internet, the contents of mailing lists have become
popular as an address information source for so-called "spammers."
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The use of EXPN to "harvest" addresses has increased as list
administrators have installed protections against inappropriate uses
of the lists themselves. However, VRFY and EXPN are still useful for
authenticated users and within an administrative domain. For
example, VRFY and EXPN are useful for performing internal audits of
how email gets routed to check and to make sure no one is
automatically forwarding sensitive mail outside the organization.
Sites implementing SMTP authentication may choose to make VRFY and
EXPN available only to authenticated requestors. Implementations
SHOULD still provide support for EXPN, but sites SHOULD carefully
evaluate the tradeoffs.
Whether disabling VRFY provides any real marginal security depends on
a series of other conditions. In many cases, RCPT commands can be
used to obtain the same information about address validity. On the
other hand, especially in situations where determination of address
validity for RCPT commands is deferred until after the DATA command
is received, RCPT may return no information at all, while VRFY is
expected to make a serious attempt to determine validity before
generating a response code (see discussion above).
7.4. Mail Rerouting Based on the 251 and 551 Response Codes
Before a client uses the 251 or 551 reply codes from a RCPT command
to automatically update its future behavior (e.g., updating the
user's address book), it should be certain of the server's
authenticity. If it does not, it may be subject to a man in the
middle attack.
7.5. Information Disclosure in Announcements
There has been an ongoing debate about the tradeoffs between the
debugging advantages of announcing server type and version (and,
sometimes, even server domain name) in the greeting response or in
response to the HELP command and the disadvantages of exposing
information that might be useful in a potential hostile attack. The
utility of the debugging information is beyond doubt. Those who
argue for making it available point out that it is far better to
actually secure an SMTP server rather than hope that trying to
conceal known vulnerabilities by hiding the server's precise identity
will provide more protection. Sites are encouraged to evaluate the
tradeoff with that issue in mind; implementations SHOULD minimally
provide for making type and version information available in some way
to other network hosts.
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7.6. Information Disclosure in Trace Fields
In some circumstances, such as when mail originates from within a LAN
whose hosts are not directly on the public Internet, trace
("Received") header fields produced in conformance with this
specification may disclose host names and similar information that
would not normally be available. This ordinarily does not pose a
problem, but sites with special concerns about name disclosure should
be aware of it. Also, the optional FOR clause should be supplied
with caution or not at all when multiple recipients are involved lest
it inadvertently disclose the identities of "blind copy" recipients
to others.
7.7. Information Disclosure in Message Forwarding
As discussed in Section 3.4, use of the 251 or 551 reply codes to
identify the replacement address associated with a mailbox may
inadvertently disclose sensitive information. Sites that are
concerned about those issues should ensure that they select and
configure servers appropriately.
7.8. Resistance to Attacks
In recent years, there has been an increase of attacks on SMTP
servers, either in conjunction with attempts to discover addresses
for sending unsolicited messages or simply to make the servers
inaccessible to others (i.e., as an application-level denial of
service attack). While the means of doing so are beyond the scope of
this Standard, rational operational behavior requires that servers be
permitted to detect such attacks and take action to defend
themselves. For example, if a server determines that a large number
of RCPT TO commands are being sent, most or all with invalid
addresses, as part of such an attack, it would be reasonable for the
server to close the connection after generating an appropriate number
of 5yz (normally 550) replies.
7.9. Scope of Operation of SMTP Servers
It is a well-established principle that an SMTP server may refuse to
accept mail for any operational or technical reason that makes sense
to the site providing the server. However, cooperation among sites
and installations makes the Internet possible. If sites take
excessive advantage of the right to reject traffic, the ubiquity of
email availability (one of the strengths of the Internet) will be
threatened; considerable care should be taken and balance maintained
if a site decides to be selective about the traffic it will accept
and process.
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In recent years, use of the relay function through arbitrary sites
has been used as part of hostile efforts to hide the actual origins
of mail. Some sites have decided to limit the use of the relay
function to known or identifiable sources, and implementations SHOULD
provide the capability to perform this type of filtering. When mail
is rejected for these or other policy reasons, a 550 code SHOULD be
used in response to EHLO (or HELO), MAIL, or RCPT as appropriate.
8. IANA Considerations
IANA maintains three registries in support of this specification, all
of which were created for RFC 2821 or earlier. This document expands
the third one as specified below. The registry references listed are
as of the time of publication; IANA does not guarantee the locations
associated with the URLs. The registries are as follows:
o The first, "Simple Mail Transfer Protocol (SMTP) Service
Extensions" [46], consists of SMTP service extensions with the
associated keywords, and, as needed, parameters and verbs. As
specified in Section 2.2.2, no entry may be made in this registry
that starts in an "X". Entries may be made only for service
extensions (and associated keywords, parameters, or verbs) that
are defined in Standards-Track or Experimental RFCs specifically
approved by the IESG for this purpose.
o The second registry, "Address Literal Tags" [47], consists of
"tags" that identify forms of domain literals other than those for
IPv4 addresses (specified in RFC 821 and in this document). The
initial entry in that registry is for IPv6 addresses (specified in
this document). Additional literal types require standardization
before being used; none are anticipated at this time.
o The third, "Mail Transmission Types" [46], established by RFC 821
and renewed by this specification, is a registry of link and
protocol identifiers to be used with the "via" and "with"
subclauses of the time stamp ("Received:" header field) described
in Section 4.4. Link and protocol identifiers in addition to
those specified in this document may be registered only by
standardization or by way of an RFC-documented, IESG-approved,
Experimental protocol extension. This name space is for
identification and not limited in size: the IESG is encouraged to
approve on the basis of clear documentation and a distinct method
rather than preferences about the properties of the method itself.
An additional subsection has been added to the "VIA link types"
and "WITH protocol types" subsections of this registry to contain
registrations of "Additional-registered-clauses" as described
above. The registry will contain clause names, a description, a
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summary of the syntax of the associated String, and a reference.
As new clauses are defined, they may, in principle, specify
creation of their own registries if the Strings consist of
reserved terms or keywords rather than less restricted strings.
As with link and protocol identifiers, additional clauses may be
registered only by standardization or by way of an RFC-documented,
IESG-approved, Experimental protocol extension. The additional
clause name space is for identification and is not limited in
size: the IESG is encouraged to approve on the basis of clear
documentation, actual use or strong signs that the clause will be
used, and a distinct requirement rather than preferences about the
properties of the clause itself.
In addition, if additional trace header fields (i.e., in addition to
Return-path and Received) are ever created, those trace fields MUST
be added to the IANA registry established by BCP 90 (RFC 3864) [11]
for use with RFC 5322 [4].
9. Acknowledgments
Many people contributed to the development of RFC 2821. That
document should be consulted for those acknowledgments. For the
present document, the editor and the community owe thanks to Dawn
Mann and Tony Hansen who assisted in the very painful process of
editing and converting the internal format of the document from one
system to another.
Neither this document nor RFC 2821 would have been possible without
the many contribution and insights of the late Jon Postel. Those
contributions of course include the original specification of SMTP in
RFC 821. A considerable quantity of text from RFC 821 still appears
in this document as do several of Jon's original examples that have
been updated only as needed to reflect other changes in the
specification.
Many people made comments or suggestions on the mailing list or in
notes to the author. Important corrections or clarifications were
suggested by several people, including Matti Aarnio, Glenn Anderson,
Derek J. Balling, Alex van den Bogaerdt, Stephane Bortzmeyer, Vint
Cerf, Jutta Degener, Steve Dorner, Lisa Dusseault, Frank Ellerman,
Ned Freed, Randy Gellens, Sabahattin Gucukoglu, Philip Guenther, Arnt
Gulbrandsen, Eric Hall, Richard O. Hammer, Tony Hansen, Peter J.
Holzer, Kari Hurtta, Bryon Roche Kain, Valdis Kletnieks, Mathias
Koerber, John Leslie, Bruce Lilly, Jeff Macdonald, Mark E. Mallett,
Mark Martinec, S. Moonesamy, Lyndon Nerenberg, Chris Newman, Douglas
Otis, Pete Resnick, Robert A. Rosenberg, Vince Sabio, Hector Santos,
David F. Skoll, Paul Smith, and Brett Watson.
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The efforts of the Area Directors -- Lisa Dusseault, Ted Hardie, and
Chris Newman -- to get this effort restarted and keep it moving, and
of an ad hoc committee with the same purpose, are gratefully
acknowledged. The members of that committee were (in alphabetical
order) Dave Crocker, Cyrus Daboo, Tony Finch, Ned Freed, Randall
Gellens, Tony Hansen, the author, and Alexey Melnikov. Tony Hansen
also acted as ad hoc chair on the mailing list reviewing this
document; without his efforts, sense of balance and fairness, and
patience, it clearly would not have been possible.
10. References
10.1. Normative References
[1] Postel, J., "Simple Mail Transfer Protocol", STD 10, RFC 821,
August 1982.
[2] Mockapetris, P., "Domain names - implementation and
specification", STD 13, RFC 1035, November 1987.
[3] Braden, R., "Requirements for Internet Hosts - Application and
Support", STD 3, RFC 1123, October 1989.
[4] Resnick, P., "Internet Message Format", RFC 5322, October 2008.
[5] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
[6] American National Standards Institute (formerly United States
of America Standards Institute), "USA Code for Information
Interchange", ANSI X3.4-1968, 1968.
ANSI X3.4-1968 has been replaced by newer versions with slight
modifications, but the 1968 version remains definitive for the
Internet.
[7] Crocker, D. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234, January 2008.
[8] Hinden, R. and S. Deering, "IP Version 6 Addressing
Architecture", RFC 4291, February 2006.
[9] Newman, C., "ESMTP and LMTP Transmission Types Registration",
RFC 3848, July 2004.
[10] Klensin, J., Freed, N., and K. Moore, "SMTP Service Extension
for Message Size Declaration", STD 10, RFC 1870, November 1995.
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RFC 5321 SMTP October 2008
[11] Klyne, G., Nottingham, M., and J. Mogul, "Registration
Procedures for Message Header Fields", BCP 90, RFC 3864,
September 2004.
10.2. Informative References
[12] Partridge, C., "Mail routing and the domain system", RFC 974,
January 1986.
[13] Klensin, J., Freed, N., Rose, M., Stefferud, E., and D.
Crocker, "SMTP Service Extensions", STD 10, RFC 1869,
November 1995.
[14] Klensin, J., "Simple Mail Transfer Protocol", RFC 2821,
April 2001.
[15] Butler, M., Postel, J., Chase, D., Goldberger, J., and J.
Reynolds, "Post Office Protocol: Version 2", RFC 937,
February 1985.
[16] Myers, J. and M. Rose, "Post Office Protocol - Version 3",
STD 53, RFC 1939, May 1996.
[17] Crispin, M., "INTERNET MESSAGE ACCESS PROTOCOL - VERSION
4rev1", RFC 3501, March 2003.
[18] Gellens, R. and J. Klensin, "Message Submission for Mail",
RFC 4409, April 2006.
[19] Freed, N., "SMTP Service Extension for Command Pipelining",
STD 60, RFC 2920, September 2000.
[20] Vaudreuil, G., "SMTP Service Extensions for Transmission of
Large and Binary MIME Messages", RFC 3030, December 2000.
[21] Freed, N. and N. Borenstein, "Multipurpose Internet Mail
Extensions (MIME) Part One: Format of Internet Message Bodies",
RFC 2045, November 1996.
[22] Klensin, J., Freed, N., Rose, M., Stefferud, E., and D.
Crocker, "SMTP Service Extension for 8bit-MIMEtransport",
RFC 1652, July 1994.
[23] Moore, K., "MIME (Multipurpose Internet Mail Extensions) Part
Three: Message Header Extensions for Non-ASCII Text", RFC 2047,
November 1996.
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[24] Freed, N. and K. Moore, "MIME Parameter Value and Encoded Word
Extensions: Character Sets, Languages, and Continuations",
RFC 2231, November 1997.
[25] Vaudreuil, G., "Enhanced Mail System Status Codes", RFC 3463,
January 2003.
[26] Hansen, T. and J. Klensin, "A Registry for SMTP Enhanced Mail
System Status Codes", BCP 138, RFC 5248, June 2008.
[27] Freed, N., "Behavior of and Requirements for Internet
Firewalls", RFC 2979, October 2000.
[28] Crocker, D., "Standard for the format of ARPA Internet text
messages", STD 11, RFC 822, August 1982.
[29] Wong, M. and W. Schlitt, "Sender Policy Framework (SPF) for
Authorizing Use of Domains in E-Mail, Version 1", RFC 4408,
April 2006.
[30] Fenton, J., "Analysis of Threats Motivating DomainKeys
Identified Mail (DKIM)", RFC 4686, September 2006.
[31] Allman, E., Callas, J., Delany, M., Libbey, M., Fenton, J., and
M. Thomas, "DomainKeys Identified Mail (DKIM) Signatures",
RFC 4871, May 2007.
[32] Moore, K., "Simple Mail Transfer Protocol (SMTP) Service
Extension for Delivery Status Notifications (DSNs)", RFC 3461,
January 2003.
[33] Moore, K. and G. Vaudreuil, "An Extensible Message Format for
Delivery Status Notifications", RFC 3464, January 2003.
[34] Postel, J. and J. Reynolds, "File Transfer Protocol", STD 9,
RFC 959, October 1985.
[35] Kille, S., "MIXER (Mime Internet X.400 Enhanced Relay): Mapping
between X.400 and RFC 822/MIME", RFC 2156, January 1998.
[36] De Winter, J., "SMTP Service Extension for Remote Message Queue
Starting", RFC 1985, August 1996.
[37] Hansen, T. and G. Vaudreuil, "Message Disposition
Notification", RFC 3798, May 2004.
[38] Elz, R. and R. Bush, "Clarifications to the DNS Specification",
RFC 2181, July 1997.
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[39] Nakamura, M. and J. Hagino, "SMTP Operational Experience in
Mixed IPv4/v6 Environments", RFC 3974, January 2005.
[40] Partridge, C., "Duplicate messages and SMTP", RFC 1047,
February 1988.
[41] Crispin, M., "Interactive Mail Access Protocol: Version 2",
RFC 1176, August 1990.
[42] Lambert, M., "PCMAIL: A distributed mail system for personal
computers", RFC 1056, June 1988.
[43] Galvin, J., Murphy, S., Crocker, S., and N. Freed, "Security
Multiparts for MIME: Multipart/Signed and Multipart/Encrypted",
RFC 1847, October 1995.
[44] Callas, J., Donnerhacke, L., Finney, H., Shaw, D., and R.
Thayer, "OpenPGP Message Format", RFC 4880, November 2007.
[45] Ramsdell, B., "Secure/Multipurpose Internet Mail Extensions
(S/MIME) Version 3.1 Message Specification", RFC 3851,
July 2004.
[46] Internet Assigned Number Authority (IANA), "IANA Mail
Parameters", 2007,
<http://www.iana.org/assignments/mail-parameters>.
[47] Internet Assigned Number Authority (IANA), "Address Literal
Tags", 2007,
<http://www.iana.org/assignments/address-literal-tags>.
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Appendix A. TCP Transport Service
The TCP connection supports the transmission of 8-bit bytes. The
SMTP data is 7-bit ASCII characters. Each character is transmitted
as an 8-bit byte with the high-order bit cleared to zero. Service
extensions may modify this rule to permit transmission of full 8-bit
data bytes as part of the message body, or, if specifically designed
to do so, in SMTP commands or responses.
Appendix B. Generating SMTP Commands from RFC 822 Header Fields
Some systems use an RFC 822 header section (only) in a mail
submission protocol, or otherwise generate SMTP commands from RFC 822
header fields when such a message is handed to an MTA from a UA.
While the MTA-UA protocol is a private matter, not covered by any
Internet Standard, there are problems with this approach. For
example, there have been repeated problems with proper handling of
"bcc" copies and redistribution lists when information that
conceptually belongs to the mail envelope is not separated early in
processing from header field information (and kept separate).
It is recommended that the UA provide its initial ("submission
client") MTA with an envelope separate from the message itself.
However, if the envelope is not supplied, SMTP commands SHOULD be
generated as follows:
1. Each recipient address from a TO, CC, or BCC header field SHOULD
be copied to a RCPT command (generating multiple message copies
if that is required for queuing or delivery). This includes any
addresses listed in a RFC 822 "group". Any BCC header fields
SHOULD then be removed from the header section. Once this
process is completed, the remaining header fields SHOULD be
checked to verify that at least one TO, CC, or BCC header field
remains. If none do, then a BCC header field with no additional
information SHOULD be inserted as specified in [4].
2. The return address in the MAIL command SHOULD, if possible, be
derived from the system's identity for the submitting (local)
user, and the "From:" header field otherwise. If there is a
system identity available, it SHOULD also be copied to the Sender
header field if it is different from the address in the From
header field. (Any Sender header field that was already there
SHOULD be removed.) Systems may provide a way for submitters to
override the envelope return address, but may want to restrict
its use to privileged users. This will not prevent mail forgery,
but may lessen its incidence; see Section 7.1.
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When an MTA is being used in this way, it bears responsibility for
ensuring that the message being transmitted is valid. The mechanisms
for checking that validity, and for handling (or returning) messages
that are not valid at the time of arrival, are part of the MUA-MTA
interface and not covered by this specification.
A submission protocol based on Standard RFC 822 information alone
MUST NOT be used to gateway a message from a foreign (non-SMTP) mail
system into an SMTP environment. Additional information to construct
an envelope must come from some source in the other environment,
whether supplemental header fields or the foreign system's envelope.
Attempts to gateway messages using only their header "To" and "Cc"
fields have repeatedly caused mail loops and other behavior adverse
to the proper functioning of the Internet mail environment. These
problems have been especially common when the message originates from
an Internet mailing list and is distributed into the foreign
environment using envelope information. When these messages are then
processed by a header-section-only remailer, loops back to the
Internet environment (and the mailing list) are almost inevitable.
Appendix C. Source Routes
Historically, the <reverse-path> was a reverse source routing list of
hosts and a source mailbox. The first host in the <reverse-path> was
historically the host sending the MAIL command; today, source routes
SHOULD NOT appear in the reverse-path. Similarly, the <forward-path>
may be a source routing lists of hosts and a destination mailbox.
However, in general, the <forward-path> SHOULD contain only a mailbox
and domain name, relying on the domain name system to supply routing
information if required. The use of source routes is deprecated (see
Appendix F.2); while servers MUST be prepared to receive and handle
them as discussed in Section 3.3 and Appendix F.2, clients SHOULD NOT
transmit them and this section is included in the current
specification only to provide context. It has been modified somewhat
from the material in RFC 821 to prevent server actions that might
confuse clients or subsequent servers that do not expect a full
source route implementation.
For relay purposes, the forward-path may be a source route of the
form "@ONE,@TWO:JOE@THREE", where ONE, TWO, and THREE MUST be fully-
qualified domain names. This form is used to emphasize the
distinction between an address and a route. The mailbox (here, JOE@
THREE) is an absolute address, and the route is information about how
to get there. The two concepts should not be confused.
If source routes are used, RFC 821 and the text below should be
consulted for the mechanisms for constructing and updating the
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forward-path. A server that is reached by means of a source route
(e.g., its domain name appears first in the list in the forward-path)
MUST remove its domain name from any forward-paths in which that
domain name appears before forwarding the message and MAY remove all
other source routing information. The reverse-path SHOULD NOT be
updated by servers conforming to this specification.
Notice that the forward-path and reverse-path appear in the SMTP
commands and replies, but not necessarily in the message. That is,
there is no need for these paths and especially this syntax to appear
in the "To:" , "From:", "CC:", etc. fields of the message header
section. Conversely, SMTP servers MUST NOT derive final message
routing information from message header fields.
When the list of hosts is present despite the recommendations above,
it is a "reverse" source route and indicates that the mail was
relayed through each host on the list (the first host in the list was
the most recent relay). This list is used as a source route to
return non-delivery notices to the sender. If, contrary to the
recommendations here, a relay host adds itself to the beginning of
the list, it MUST use its name as known in the transport environment
to which it is relaying the mail rather than that of the transport
environment from which the mail came (if they are different). Note
that a situation could easily arise in which some relay hosts add
their names to the reverse source route and others do not, generating
discontinuities in the routing list. This is another reason why
servers needing to return a message SHOULD ignore the source route
entirely and simply use the domain as specified in the Mailbox.
Appendix D. Scenarios
This section presents complete scenarios of several types of SMTP
sessions. In the examples, "C:" indicates what is said by the SMTP
client, and "S:" indicates what is said by the SMTP server.
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D.1. A Typical SMTP Transaction Scenario
This SMTP example shows mail sent by Smith at host bar.com, and to
Jones, Green, and Brown at host foo.com. Here we assume that host
bar.com contacts host foo.com directly. The mail is accepted for
Jones and Brown. Green does not have a mailbox at host foo.com.
S: 220 foo.com Simple Mail Transfer Service Ready
C: EHLO bar.com
S: 250-foo.com greets bar.com
S: 250-8BITMIME
S: 250-SIZE
S: 250-DSN
S: 250 HELP
C: MAIL FROM:<Smith@bar.com>
S: 250 OK
C: RCPT TO:<Jones@foo.com>
S: 250 OK
C: RCPT TO:<Green@foo.com>
S: 550 No such user here
C: RCPT TO:<Brown@foo.com>
S: 250 OK
C: DATA
S: 354 Start mail input; end with <CRLF>.<CRLF>
C: Blah blah blah...
C: ...etc. etc. etc.
C: .
S: 250 OK
C: QUIT
S: 221 foo.com Service closing transmission channel
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D.2. Aborted SMTP Transaction Scenario
S: 220 foo.com Simple Mail Transfer Service Ready
C: EHLO bar.com
S: 250-foo.com greets bar.com
S: 250-8BITMIME
S: 250-SIZE
S: 250-DSN
S: 250 HELP
C: MAIL FROM:<Smith@bar.com>
S: 250 OK
C: RCPT TO:<Jones@foo.com>
S: 250 OK
C: RCPT TO:<Green@foo.com>
S: 550 No such user here
C: RSET
S: 250 OK
C: QUIT
S: 221 foo.com Service closing transmission channel
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D.3. Relayed Mail Scenario
Step 1 -- Source Host to Relay Host
The source host performs a DNS lookup on XYZ.COM (the destination
address) and finds DNS MX records specifying xyz.com as the best
preference and foo.com as a lower preference. It attempts to open a
connection to xyz.com and fails. It then opens a connection to
foo.com, with the following dialogue:
S: 220 foo.com Simple Mail Transfer Service Ready
C: EHLO bar.com
S: 250-foo.com greets bar.com
S: 250-8BITMIME
S: 250-SIZE
S: 250-DSN
S: 250 HELP
C: MAIL FROM:<JQP@bar.com>
S: 250 OK
C: RCPT TO:<Jones@XYZ.COM>
S: 250 OK
C: DATA
S: 354 Start mail input; end with <CRLF>.<CRLF>
C: Date: Thu, 21 May 1998 05:33:29 -0700
C: From: John Q. Public <JQP@bar.com>
C: Subject: The Next Meeting of the Board
C: To: Jones@xyz.com
C:
C: Bill:
C: The next meeting of the board of directors will be
C: on Tuesday.
C: John.
C: .
S: 250 OK
C: QUIT
S: 221 foo.com Service closing transmission channel
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Step 2 -- Relay Host to Destination Host
foo.com, having received the message, now does a DNS lookup on
xyz.com. It finds the same set of MX records, but cannot use the one
that points to itself (or to any other host as a worse preference).
It tries to open a connection to xyz.com itself and succeeds. Then
we have:
S: 220 xyz.com Simple Mail Transfer Service Ready
C: EHLO foo.com
S: 250 xyz.com is on the air
C: MAIL FROM:<JQP@bar.com>
S: 250 OK
C: RCPT TO:<Jones@XYZ.COM>
S: 250 OK
C: DATA
S: 354 Start mail input; end with <CRLF>.<CRLF>
C: Received: from bar.com by foo.com ; Thu, 21 May 1998
C: 05:33:29 -0700
C: Date: Thu, 21 May 1998 05:33:22 -0700
C: From: John Q. Public <JQP@bar.com>
C: Subject: The Next Meeting of the Board
C: To: Jones@xyz.com
C:
C: Bill:
C: The next meeting of the board of directors will be
C: on Tuesday.
C: John.
C: .
S: 250 OK
C: QUIT
S: 221 foo.com Service closing transmission channel
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D.4. Verifying and Sending Scenario
S: 220 foo.com Simple Mail Transfer Service Ready
C: EHLO bar.com
S: 250-foo.com greets bar.com
S: 250-8BITMIME
S: 250-SIZE
S: 250-DSN
S: 250-VRFY
S: 250 HELP
C: VRFY Crispin
S: 250 Mark Crispin <Admin.MRC@foo.com>
C: MAIL FROM:<EAK@bar.com>
S: 250 OK
C: RCPT TO:<Admin.MRC@foo.com>
S: 250 OK
C: DATA
S: 354 Start mail input; end with <CRLF>.<CRLF>
C: Blah blah blah...
C: ...etc. etc. etc.
C: .
S: 250 OK
C: QUIT
S: 221 foo.com Service closing transmission channel
Appendix E. Other Gateway Issues
In general, gateways between the Internet and other mail systems
SHOULD attempt to preserve any layering semantics across the
boundaries between the two mail systems involved. Gateway-
translation approaches that attempt to take shortcuts by mapping
(such as mapping envelope information from one system to the message
header section or body of another) have generally proven to be
inadequate in important ways. Systems translating between
environments that do not support both envelopes and a header section
and Internet mail must be written with the understanding that some
information loss is almost inevitable.
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Appendix F. Deprecated Features of RFC 821
A few features of RFC 821 have proven to be problematic and SHOULD
NOT be used in Internet mail.
F.1. TURN
This command, described in RFC 821, raises important security issues
since, in the absence of strong authentication of the host requesting
that the client and server switch roles, it can easily be used to
divert mail from its correct destination. Its use is deprecated;
SMTP systems SHOULD NOT use it unless the server can authenticate the
client.
F.2. Source Routing
RFC 821 utilized the concept of explicit source routing to get mail
from one host to another via a series of relays. The requirement to
utilize source routes in regular mail traffic was eliminated by the
introduction of the domain name system "MX" record and the last
significant justification for them was eliminated by the
introduction, in RFC 1123, of a clear requirement that addresses
following an "@" must all be fully-qualified domain names.
Consequently, the only remaining justifications for the use of source
routes are support for very old SMTP clients or MUAs and in mail
system debugging. They can, however, still be useful in the latter
circumstance and for routing mail around serious, but temporary,
problems such as problems with the relevant DNS records.
SMTP servers MUST continue to accept source route syntax as specified
in the main body of this document and in RFC 1123. They MAY, if
necessary, ignore the routes and utilize only the target domain in
the address. If they do utilize the source route, the message MUST
be sent to the first domain shown in the address. In particular, a
server MUST NOT guess at shortcuts within the source route.
Clients SHOULD NOT utilize explicit source routing except under
unusual circumstances, such as debugging or potentially relaying
around firewall or mail system configuration errors.
F.3. HELO
As discussed in Sections 3.1 and 4.1.1, EHLO SHOULD be used rather
than HELO when the server will accept the former. Servers MUST
continue to accept and process HELO in order to support older
clients.
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F.4. #-literals
RFC 821 provided for specifying an Internet address as a decimal
integer host number prefixed by a pound sign, "#". In practice, that
form has been obsolete since the introduction of TCP/IP. It is
deprecated and MUST NOT be used.
F.5. Dates and Years
When dates are inserted into messages by SMTP clients or servers
(e.g., in trace header fields), four-digit years MUST BE used. Two-
digit years are deprecated; three-digit years were never permitted in
the Internet mail system.
F.6. Sending versus Mailing
In addition to specifying a mechanism for delivering messages to
user's mailboxes, RFC 821 provided additional, optional, commands to
deliver messages directly to the user's terminal screen. These
commands (SEND, SAML, SOML) were rarely implemented, and changes in
workstation technology and the introduction of other protocols may
have rendered them obsolete even where they are implemented.
Clients SHOULD NOT provide SEND, SAML, or SOML as services. Servers
MAY implement them. If they are implemented by servers, the
implementation model specified in RFC 821 MUST be used and the
command names MUST be published in the response to the EHLO command.
Author's Address
John C. Klensin
1770 Massachusetts Ave, Suite 322
Cambridge, MA 02140
USA
EMail: john+smtp@jck.com
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Full Copyright Statement
Copyright (C) The IETF Trust (2008).
This document is subject to the rights, licenses and restrictions
contained in BCP 78, and except as set forth therein, the authors
retain all their rights.
This document and the information contained herein are provided on an
"AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE IETF TRUST AND
THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS
OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF
THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
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