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Updates:
Obsoletes:
Keywords: [--------], IMAP, Sieve, SMTP, Lemonade, mobile email, low-bandwidth efficient
Network Working Group D. Cridland, Ed.
Request for Comments: 5550 A. Melnikov, Ed.
Obsoletes: 4550 Isode Limited
Updates: 4469, 4467 S. Maes, Ed.
Category: Standards Track Oracle
August 2009
The Internet Email to
Support Diverse Service Environments (Lemonade) Profile
Abstract
This document describes a profile (a set of required extensions,
restrictions, and usage modes), dubbed Lemonade, of the IMAP, mail
submission, and Sieve protocols. This profile allows clients
(especially those that are constrained in memory, bandwidth,
processing power, or other areas) to efficiently use IMAP and
Submission to access and submit mail. This includes the ability to
forward received mail without needing to download and upload the
mail, to optimize submission, and to efficiently resynchronize in
case of loss of connectivity with the server.
The Lemonade Profile relies upon several extensions to IMAP, Sieve,
and Mail Submission protocols. The document also defines a new IMAP
extension and registers several new IMAP keywords.
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.
Copyright Notice
Copyright (c) 2009 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents in effect on the date of
publication of this document (http://trustee.ietf.org/license-info).
Please review these documents carefully, as they describe your rights
and restrictions with respect to this document.
Cridland, et al. Standards Track [Page 1]
RFC 5550 Lemonade Profile August 2009
This document may contain material from IETF Documents or IETF
Contributions published or made publicly available before November
10, 2008. The person(s) controlling the copyright in some of this
material may not have granted the IETF Trust the right to allow
modifications of such material outside the IETF Standards Process.
Without obtaining an adequate license from the person(s) controlling
the copyright in such materials, this document may not be modified
outside the IETF Standards Process, and derivative works of it may
not be created outside the IETF Standards Process, except to format
it for publication as an RFC or to translate it into languages other
than English.
Table of Contents
1. Introduction ....................................................3
2. Conventions Used in This Document ...............................4
3. Summary of the Required Support .................................4
3.1. Lemonade Submission Servers ................................4
3.2. Lemonade Message Stores ....................................5
3.3. Lemonade Message Delivery Agents ...........................7
4. Lemonade Submission Servers .....................................7
4.1. Forward without Download ...................................7
4.2. Pipelining .................................................8
4.3. DSN Support ................................................8
4.4. Message Size Declaration ...................................8
4.5. Enhanced Status Code Support ...............................8
4.6. Encryption and Compression .................................8
5. Lemonade Message Stores .........................................9
5.1. Quick Resynchronization ....................................9
5.2. Message Part Handling ......................................9
5.3. Compression ...............................................10
5.4. Notifications .............................................10
5.5. Searching and View Filters ................................12
5.6. Mailbox Handling ..........................................12
5.7. Forward without Download ..................................12
5.8. Additional IMAP Extensions ................................13
5.9. Registration of $Forwarded IMAP Keyword ...................13
5.10. Registration of $SubmitPending and $Submitted
IMAP Keywords ............................................13
5.11. Related IMAP Extensions ..................................14
6. Lemonade Message Delivery Agents ...............................14
7. Lemonade Message User Agents ...................................15
8. Forward without Download .......................................16
8.1. Motivations ...............................................16
8.2. Message Sending Overview ..................................16
8.3. Traditional Strategy ......................................17
8.4. A New Strategy ............................................18
8.5. Security Considerations for Pawn-Tickets ..................27
Cridland, et al. Standards Track [Page 2]
RFC 5550 Lemonade Profile August 2009
8.6. Copies of Sent Messages: The fcc Problem ..................27
9. Deployment Considerations ......................................28
10. Security Considerations .......................................28
10.1. Confidentiality Protection of Submitted Messages .........28
10.2. TLS ......................................................29
10.3. Additional Extensions and Deployment Models ..............29
11. IANA Considerations ...........................................30
12. Changes since RFC 4550 ........................................30
13. Acknowledgements ..............................................31
14. References ....................................................31
14.1. Normative References .....................................31
14.2. Informative References ...................................35
Appendix A. Errata ..............................................37
1. Introduction
The Lemonade Profile, or simply Lemonade, provides enhancements to
Internet email to support diverse service environments. Lemonade
mail servers provide both a Lemonade Submission Server and a Lemonade
Message Store, which are based on the existing [SUBMIT] and [IMAP]
protocols, respectively. They MAY also include a Lemonade Message
Delivery Agent, which provides delivery-time filtering services based
on [SIEVE].
This document describes the Lemonade Profile that includes:
o General common enhancements to Internet Mail, described in 5 and
4.
o "Forward without download" that describes exchanges between
Lemonade clients and servers to allow submitting new email
messages incorporating content that resides on locations external
to the client, described in Section 8.
o Quick mailbox resynchronization, described in Section 5.1.
o Extensions to support more precise, and broader, notifications
from the store in support of notifications and view filters,
described in 5.4.1 and 5.5.
o Delivery-time filtering in support of typical mail management use
cases, as described in Section 3.3.
The LEMONADE WG used the architecture shown in [LEMONADE-ARCH] to
develop the Lemonade Profile.
Cridland, et al. Standards Track [Page 3]
RFC 5550 Lemonade Profile August 2009
It is intended that the Lemonade Profile support realizations of the
OMA's mobile email enabler (MEM) (see [OMA-MEM-REQ] and
[OMA-MEM-ARCH]) using Internet Mail protocols defined by the IETF.
2. Conventions Used in This Document
In examples, "M:", "I:", and "S:" indicate lines sent by the client
Message User Agent, IMAP email server, and SMTP submit server,
respectively.
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 [KEYWORDS].
Other capitalized words are typically names of extensions or commands
-- these are uppercased for clarity only, and are case-insensitive.
This document uses terminology defined in [RFC5598]. See [RFC5598]
for further details on Email Architecture.
All examples in this document are optimized for Lemonade use and
might not represent examples of proper protocol usage for a general
use Submit/IMAP client. In particular, examples assume that Submit
and IMAP servers support all Lemonade extensions described in this
document, so they do not demonstrate fallbacks in the absence of an
extension.
3. Summary of the Required Support
3.1. Lemonade Submission Servers
Lemonade Submission Servers MUST provide a service as described in
[SUBMIT], and MUST support the following. Note that the Lemonade
Profile imposes further requirements for some cases, detailed in the
sections cited.
Cridland, et al. Standards Track [Page 4]
RFC 5550 Lemonade Profile August 2009
+---------------------+--------------------+--------------+
| SMTP extension | Reference | Requirements |
+---------------------+--------------------+--------------+
| 8BITMIME | [SMTP-8BITMIME] | [SMTP-BURL] |
| AUTH | [SMTP-AUTH] | [SUBMIT] |
| BINARYMIME | [SMTP-BINARYMIME] | Section 4.1 |
| BURL imap | [SMTP-BURL] | Section 8 |
| CHUNKING | [SMTP-BINARYMIME] | Section 4.1 |
| DSN | [SMTP-DSN] | Section 4.3 |
| ENHANCEDSTATUSCODES | [SMTP-STATUSCODES] | Section 4.5 |
| PIPELINING | [SMTP-PIPELINING] | Section 4.2 |
| SIZE | [SMTP-SIZE] | Section 4.4 |
| STARTTLS | [SMTP-TLS] | Section 4.6 |
+---------------------+--------------------+--------------+
3.2. Lemonade Message Stores
Lemonade Message Stores MUST provide a service as described in
[IMAP], and MUST support the following. Note that the Lemonade
Profile imposes further requirements for some cases, detailed in the
sections cited.
Cridland, et al. Standards Track [Page 5]
RFC 5550 Lemonade Profile August 2009
+------------------------+------------------+---------------+
| IMAP extension | Reference | Requirements |
+------------------------+------------------+---------------+
| BINARY | [IMAP-BINARY] | Section 5.2 |
| CATENATE | [IMAP-CATENATE] | Section 5.7 |
| COMPRESS=DEFLATE | [IMAP-COMPRESS] | Section 5.3 |
| CONDSTORE | [IMAP-CONDSTORE] | Section 5.1 |
| CONTEXT=SEARCH | [IMAP-CONTEXT] | Section 5.5 |
| CONTEXT=SORT | [IMAP-CONTEXT] | Section 5.5 |
| CONVERT | [IMAP-CONVERT] | Section 5.2 |
| ENABLE | [IMAP-ENABLE] | Section 5.1 |
| ESEARCH | [IMAP-ESEARCH] | Section 5.5 |
| ESORT | [IMAP-CONTEXT] | Section 5.5 |
| I18NLEVEL=1 | [IMAP-I18N] | Section 5.8 |
| IDLE | [IMAP-IDLE] | Section 5.4.1 |
| LITERAL+ | [IMAP-LITERAL+] | Section 5.8 |
| NAMESPACE | [IMAP-NAMESPACE] | Section 5.6 |
| NOTIFY | [IMAP-NOTIFY] | Section 5.4.1 |
| QRESYNC | [IMAP-QRESYNC] | Section 5.1 |
| SASL-IR | [IMAP-SASL-IR] | Section 5.8 |
| SORT | [IMAP-SORT] | Section 5.5 |
| STARTTLS | [IMAP] | - |
| UIDPLUS | [IMAP-UIDPLUS] | Section 5.7 |
| URLAUTH | [IMAP-URLAUTH] | Section 5.7 |
| URL-PARTIAL | Section 5.7.1 | Section 5.7 |
| $Forwarded keyword | - | Section 5.9 |
| $SubmitPending keyword | - | Section 5.10 |
| $Submitted keyword | - | Section 5.10 |
+------------------------+------------------+---------------+
In addition to this list, any Lemonade Message Stores MUST send the
CAPABILITY response code (see Section 7.1 of [IMAP]) in the initial
server greeting and after the LOGIN/AUTHENTICATE commands.
Cridland, et al. Standards Track [Page 6]
RFC 5550 Lemonade Profile August 2009
3.3. Lemonade Message Delivery Agents
Lemonade Message Delivery Agents MUST support Sieve mail filtering
language as described in [SIEVE], and MUST support the following
Sieve extensions. Note that the Lemonade Profile imposes further
requirements for some cases, detailed in the sections cited.
+------------------------------+--------------------+--------------+
| Sieve extension | Reference | Requirements |
+------------------------------+--------------------+--------------+
| ENOTIFY | [SIEVE-NOTIFY] | Section 6 |
| IMAP4FLAGS | [SIEVE-IMAP4FLAGS] | Section 6 |
| RELATIONAL | [SIEVE-RELATIONAL] | Section 6 |
| VACATION | [SIEVE-VACATION] | Section 6 |
| VARIABLES | [SIEVE-VARIABLES] | Section 6 |
| comparator-i;unicode-casemap | [UNICODE-CASEMAP] | Section 6 |
+------------------------------+--------------------+--------------+
Lemonade Message Delivery Agents should also consider supporting a
Sieve script management protocol, such as [MANAGESIEVE].
4. Lemonade Submission Servers
All Lemonade Submission Servers implement the Mail Submission
protocol described in [SUBMIT], which is in turn a specific profile
of [ESMTP]. Therefore, any MUA designed to submit email via [SUBMIT]
or [ESMTP] will interoperate with Lemonade Submission Servers.
In addition, Lemonade Submission Servers implement the following set
of SMTP and Submission extensions to increase message submission
efficiency.
4.1. Forward without Download
In order to optimize network usage for the typical case where message
content is copied to, or sourced from, the IMAP store, Lemonade
provides support for a suite of extensions collectively known as
"forward without download", discussed in detail in Section 8.
Lemonade Submission Servers MUST support BURL [SMTP-BURL], 8BITMIME
[SMTP-8BITMIME], BINARYMIME [SMTP-BINARYMIME], and CHUNKING
[SMTP-BINARYMIME] SMTP extensions.
BURL MUST support URLAUTH type URLs [IMAP-URLAUTH], and thus MUST
advertise the "imap" option following the BURL EHLO keyword (see
[SMTP-BURL] for more details).
Cridland, et al. Standards Track [Page 7]
RFC 5550 Lemonade Profile August 2009
4.2. Pipelining
Some clients regularly use networks with a relatively high latency,
such as mobile or satellite-based networks. Avoidance of round trips
within a transaction has a great advantage for the reduction in both
bandwidth and total transaction time. For this reason, Lemonade-
compliant mail Submission Servers MUST support the SMTP service
extensions for command pipelining [SMTP-PIPELINING].
4.3. DSN Support
Lemonade-compliant mail Submission Servers MUST support SMTP service
extensions for delivery status notifications [SMTP-DSN].
4.4. Message Size Declaration
There is a distinct advantage in detecting failure cases as early as
possible in many cases, such as where the user is charged per octet,
or where bandwidth is low. This is especially true of large message
sizes.
Lemonade Submission Servers MUST support the SMTP service extension
for message size declaration [SMTP-SIZE].
Lemonade Submission Servers MUST expand all BURL parts before
evaluating if the supplied message size is acceptable.
A Lemonade-capable client SHOULD use message size declaration. In
particular, the client MUST NOT send a message to a mail Submission
Server if it knows that the message exceeds the maximal message size
advertised by the Submission Server. When including a message size
in the MAIL FROM command, the client MUST use a value that is at
least as large as the size of the assembled message data after
resolution of all BURL parts.
4.5. Enhanced Status Code Support
Lemonade-compliant mail Submission Servers MUST support the SMTP
service extension for returning enhanced error codes
[SMTP-STATUSCODES]. These allow a client to determine the precise
cause of failure.
4.6. Encryption and Compression
Lemonade-compliant mail Submission Servers MUST support the SMTP
service extension for secure SMTP over Transport Layer Security (TLS)
[SMTP-TLS].
Cridland, et al. Standards Track [Page 8]
RFC 5550 Lemonade Profile August 2009
Support for the DEFLATE compression method, as described in
[TLS-COMP], is RECOMMENDED.
5. Lemonade Message Stores
All Lemonade Message Stores implement the Internet Message Access
Protocol, as defined in [IMAP]. Therefore, any MUA written to access
messages using the facilities described in [IMAP] will interoperate
with a Lemonade Message Store.
In addition, Lemonade Message Stores provide a set of extensions to
address the limitations of some clients and networks.
5.1. Quick Resynchronization
Resynchronization is a costly part of an IMAP session, and mobile
networks are generally more prone to unintended disconnection, which
in turn makes this problem more acute. Therefore, Lemonade Message
Stores provide a suite of extensions to reduce the synchronization
cost.
Lemonade-compliant IMAP servers MUST support the CONDSTORE
[IMAP-CONDSTORE], the QRESYNC [IMAP-QRESYNC], and the ENABLE
[IMAP-ENABLE] extensions. These allow a client to quickly
resynchronize any mailbox by asking the server to return all flag
changes and expunges that have occurred since a previously recorded
state. This can also speed up client reconnect in case the transport
layer is cut, whether accidentally or as part of a change in network.
When implementing QRESYNC [IMAP-QRESYNC], client and servers need to
also comply with errata submitted for this document (see Appendix A).
[IMAP-SYNC-HOWTO] details how clients perform efficient mailbox
resynchronization.
5.2. Message Part Handling
The handling of message parts, especially attachments, represents a
set of challenges to limited devices, both in terms of the bandwidth
used and the capability of the device.
Lemonade-compliant IMAP servers MUST support the BINARY [IMAP-BINARY]
extension. This moves MIME body part decoding operations from the
client to the server. The decoded data is equal to or less in size
than the encoded representation, so this reduces bandwidth
effectively.
Cridland, et al. Standards Track [Page 9]
RFC 5550 Lemonade Profile August 2009
[IMAP-BINARY] allows for servers to refuse to accept uploaded
messages containing binary data, by not accepting the Binary content-
transfer-encoding; however, Lemonade-compliant IMAP servers SHALL
always accept binary encoded MIME messages in APPEND commands for any
folder.
[IMAP-CONVERT] MUST also be supported by servers, which allows
clients to request conversions between media types, and allows for
scaling images, etc. This provides the ability to view attachments
(and sometimes body parts) without the facility to cope with a wide
range of media types, or to efficiently view attachments.
5.3. Compression
Lemonade Message Stores SHOULD support the Deflate compression
algorithm for TLS, as defined in [TLS-COMP], in order to facilitate
compression at as low a level as possible.
However, the working group acknowledges that for many endpoints, this
is a rarely deployed technology, and as such, Lemonade Message Stores
MUST provide [IMAP-COMPRESS] support for fallback application-level
stream compression, where TLS is not actively providing compression.
5.4. Notifications
The addition of server-to-client notifications transforms the
Lemonade Profile into an event-based synchronization protocol.
Whenever an event occurs that interests the MUA, a notification can
be generated. The Lemonade WG used the notifications architecture
shown in [LEMONADE-NOTIFICATIONS] to develop the Lemonade Profile.
If the MUA is connected to the IMAP server, inband notifications are
generated using the facilities outlined in Section 5.4.1.
When the MUA is not connected, the notification filter generates an
outband notification. The notification filter may be considered as
acting on a push email repository.
If the MUA is not connected, and outband notification is disabled,
the client must perform a quick-sync on reconnect to determine
mailbox changes, using the mechanisms outlined in Section 5.1.
Cridland, et al. Standards Track [Page 10]
RFC 5550 Lemonade Profile August 2009
5.4.1. IMAP Notifications
Lemonade Message Stores MUST support the IDLE [IMAP-IDLE] extension.
The extension allows clients to receive unsolicited notifications
about changes in the selected mailbox, without needing to poll for
changes. The responses forming these notifications MUST be sent in a
timely manner when such changes happen.
Lemonade Message Stores also provide the NOTIFY extension described
in [IMAP-NOTIFY], which allows clients to request specific event
types to be sent immediately to the client, both for the currently
selected folder and others. Such event types include message
delivery and mailbox renames.
5.4.2. External Notifications
Lemonade and TCP provide for long-lived idle connections between the
client and mail store, allowing the server to push notifications
within IMAP. Some mobile networks support dormancy, which shuts down
the radio traffic channel during idle periods to conserve handset and
network resources, while maintaining IP and TCP state. (See the
[LEMONADE-DEPLOYMENTS] document for more information.)
However, there are environments where the email client cannot remain
active indefinitely, or where it is not advisable (or even always
possible) for TCP connections to the server to remain up while idle
for extended periods. In these situations, a good user experience
requires that when "interesting" events occur in the mail store, the
client be informed so that it can connect and resynchronize. At an
absolute minimum, this requires that at least the arrival of new mail
generate some sort of wake-up to the email client. A number of
vendors have implemented various solutions to this. As examples of
what has been done, for many years (long pre-dating cellular
handsets) the technique described in [FINGER-HACK] has been
supported. Today, a number of email vendors include facilities to
send SMS or other simple non-stream messages to clients on handsets
when new mail arrives. The Open Mobile Alliance (OMA) has published
a mechanism that uses WAP PUSH to send a basic message containing a
URL [OMA-EMN]. The IETF is investigating ways to standardize
enhanced functionality in this area.
A "push email" user experience can be achieved using any number of
techniques, ranging from always-on TCP connectivity to the server and
the NOTIFY extension described above, to OMA EMN, or even a non-
standard trigger message over SMS. In any technique, the client
learns of the existence of new mail, and decides to fetch information
about it, some part of it, or all of it, and then presents this to
the user.
Cridland, et al. Standards Track [Page 11]
RFC 5550 Lemonade Profile August 2009
5.5. Searching and View Filters
Lemonade Message Stores MUST support the ESEARCH [IMAP-ESEARCH]
extension. The extension allows clients to efficiently find the
first or last messages, find a count of matching messages, and obtain
a list of matching messages in a considerably more compact
representation.
Lemonade Message Stores also provide a mechanism for clients to avoid
handling an entire mailbox, instead accessing a view of the mailbox.
This technique, common in many desktop clients as a client-side
capability, is useful for constrained clients to minimize the
quantity of messages and notification data.
Lemonade Message Stores therefore MUST implement the CONTEXT=SEARCH,
ESORT, and CONTEXT=SORT extensions defined in [IMAP-CONTEXT], as well
as the SORT extension defined in [IMAP-SORT].
5.6. Mailbox Handling
Lemonade Message Stores MUST support the NAMESPACE [IMAP-NAMESPACE]
extension. The extension allows clients to discover shared mailboxes
and mailboxes belonging to other users, and provide a normalized
hierarchy view of the mailboxes available.
Lemonade Message Stores MUST support the I18NLEVEL=<n> [IMAP-I18N]
extension, with <n> having the value 1 or 2. It adds support for
non-English (internationalized) search and sort functions. (Note
that I18NLEVEL=2 implies support for I18NLEVEL=1, so a Lemonade-
compliant client that makes use of this extension MUST recognize
either one.)
5.7. Forward without Download
In order to optimize network usage for the typical case where message
content is copied to, or sourced from, the IMAP store, Lemonade
provides support for a suite of extensions collectively known as
"forward without download", discussed in detail in Section 8.
Lemonade Message Stores MUST support CATENATE [IMAP-CATENATE],
UIDPLUS [IMAP-UIDPLUS], and URLAUTH [IMAP-URLAUTH]. Lemonade Message
Stores MUST also support URL-PARTIAL as described in Section 5.7.1.
5.7.1. Support for PARTIAL in CATENATE and URLAUTH
[IMAP-URL] introduced a new syntactic element for referencing a byte
range of a message/body part. This is done using the ;PARTIAL=
field. If an IMAP server supports PARTIAL in IMAP URL used in
Cridland, et al. Standards Track [Page 12]
RFC 5550 Lemonade Profile August 2009
CATENATE and URLAUTH extensions, then it MUST advertise the URL-
PARTIAL capability in both the CAPABILITY response and the equivalent
response-code.
5.8. Additional IMAP Extensions
Lemonade Message Stores MUST support the LITERAL+ [IMAP-LITERAL+]
extension. The extension allows clients to save a round trip each
time a non-synchronizing literal is sent.
Lemonade Message Stores MUST also implement the SASL-IR
[IMAP-SASL-IR] extension, which allows clients to save a round trip
during authentication, potentially pipelining the entire
authentication sequence.
Lemonade-compliant IMAP servers MUST support IMAP over TLS [IMAP] as
required by [IMAP]. As noted above in Section 5.3, servers SHOULD
support the deflate compression algorithm for TLS, as specified in
[TLS-COMP].
5.9. Registration of $Forwarded IMAP Keyword
The $Forwarded IMAP keyword is used by several IMAP clients to
specify that the marked message was forwarded to another email
address, embedded within or attached to a new message. A mail client
sets this keyword when it successfully forwards the message to
another email address. Typical usage of this keyword is to show a
different (or additional) icon for a message that has been forwarded.
Once set, the flag SHOULD NOT be cleared.
Lemonade Message Stores MUST be able to store the $Forwarded keyword.
They MUST preserve it on the COPY operation. The servers MUST
support the SEARCH KEYWORD $Forwarded.
5.10. Registration of $SubmitPending and $Submitted IMAP Keywords
The $SubmitPending IMAP keyword designates the message as awaiting to
be submitted. This keyword allows storing messages waiting to be
submitted in the same mailbox where messages that were already
submitted and/or are being edited are stored. A mail client sets
this keyword when it decides that the message needs to be sent out.
When a client (it might be a different client from the one that
decided that the message is pending submission) starts sending the
message, it atomically (using "STORE (UNCHANGEDSINCE)") adds the
$Submitted keyword. Once submission is successful, the
$SubmitPending keyword is atomically cleared. The two keywords allow
messages being actively submitted (messages that have both $Submitted
and $SubmitPending keywords set) to be distinguished from messages
Cridland, et al. Standards Track [Page 13]
RFC 5550 Lemonade Profile August 2009
awaiting to be submitted, or from messages already submitted. They
also allow all messages that were supposed to be submitted to be
found, if the client submitting them crashes or quits before
submitting them.
Lemonade Message Stores MUST be able to store the $SubmitPending and
the $Submitted keyword. Lemonade Message Stores MUST preserve them
on the COPY operation. The servers MUST support the SEARCH KEYWORD
$SubmitPending and SEARCH KEYWORD $Submitted.
5.11. Related IMAP Extensions
Section 5.11 is non-normative.
Server implementations targeting to fulfill OMA MEM requirements
[OMA-MEM-REQ] should consider implementing the [IMAP-FILTERS], which
provides a way to persist definition of virtual mailboxes on the
server. They should also consider implementing the METADATA-SERVER
[METADATA] extension, which provides a way of storing user-defined
data associated with a user account.
6. Lemonade Message Delivery Agents
Lemonade Message Delivery Agents MUST support the [SIEVE] filtering
language at the point of delivery, allowing the user to control which
messages are accepted, and where they are filed.
Lemonade Message Delivery Agents MUST support the Sieve Vacation
extension [SIEVE-VACATION], which allows the client to set up an
auto-responder, typically to report being on vacation (thus the name
of the Sieve extension).
Lemonade Message Delivery Agents MUST support the Sieve Enotify
extension [SIEVE-NOTIFY], which allows a Sieve script to generate
notifications (such as XMPP, SIP, or email) about received messages.
Lemonade Message Delivery Agents MUST support the Sieve Variables
extension [SIEVE-VARIABLES], which adds support for variables to the
Sieve scripting language. This extension is typically used with
Sieve Enotify or Vacation to customize responses/notifications.
Lemonade Message Delivery Agents MUST support the Sieve Relational
extension [SIEVE-RELATIONAL], which adds support for relational
comparisons to the Sieve scripting language. This extension is
typically used together with Sieve Enotify.
Cridland, et al. Standards Track [Page 14]
RFC 5550 Lemonade Profile August 2009
Lemonade Message Delivery Agents MUST support the Sieve Imap4Flags
extension [SIEVE-IMAP4FLAGS], which allows a Sieve script to set IMAP
flags/keywords when delivering a message to a mailbox. For example,
this can be used to automatically mark certain messages as
interesting, urgent, etc.
Lemonade Message Delivery Agents MUST support the i;unicode-casemap
comparator in Sieve [UNICODE-CASEMAP], which is declared as
"comparator-i;unicode-casemap" in the Sieve "require" statement. The
comparator allows for case-insensitive matching of Unicode
characters.
Lemonade Message Delivery Agents should consider supporting Sieve
script management using the [MANAGESIEVE] protocol. If they do, they
MUST also advertise in [MANAGESIEVE] all Sieve extensions listed in
this section.
7. Lemonade Message User Agents
Although all existing IMAP MUAs are Lemonade compliant in as much as
all Lemonade services are based on the existing [IMAP] and [SUBMIT]
protocols, client implementors are encouraged to take full advantage
of the facilities provided by Lemonade Submission Servers and
Lemonade Message Stores, as described in 4 and 5, respectively.
When opening a connection to the Submission Server, clients MUST do
so using port 587 unless explicitly configured to use an alternate
port [RFC5068]. (Note that this requirement is somewhat stronger
than the one specified in [SUBMIT], as [SUBMIT] didn't prescribe the
exact procedure to be used by submission clients.) If the TCP
connection to the submission server fails to open using port 587, the
client MAY then immediately retry using a different port, such as 25.
See [SUBMIT] for information on why using port 25 is likely to fail
depending on the current location of the client, and may result in a
failure code during the SMTP transaction.
In addition, some specifications are useful to support interoperable
messaging with an enhanced user experience.
Lemonade-capable clients SHOULD support the Format and DelSp
parameters to the text/plain media type described in [FLOWED], and
generate this format for messages.
Lemonade-capable clients SHOULD support, and use, the $Forwarded
keyword described in Section 5.9.
Cridland, et al. Standards Track [Page 15]
RFC 5550 Lemonade Profile August 2009
8. Forward without Download
8.1. Motivations
The advent of client/server email using the [IMAP] and [SUBMIT]
protocols changed what formerly were local disk operations to become
repetitive network data transmissions.
Lemonade "forward without download" makes use of the [SMTP-BURL]
extension to enable access to external sources during the submission
of a message. In combination with the [IMAP-URLAUTH] extension,
inclusion of message parts or even entire messages from the IMAP mail
store is possible with a minimal trust relationship between the IMAP
and SMTP SUBMIT servers.
Lemonade "forward without download" has the advantage of maintaining
one submission protocol, and thus avoids the risk of having multiple
parallel and possibly divergent mechanisms for submission. The
client can use [SUBMIT] extensions without these being added to IMAP.
Furthermore, by keeping the details of message submission in the SMTP
SUBMIT server, Lemonade "forward without download" can work with
other message retrieval protocols such as POP, NNTP, or whatever else
may be designed in the future.
8.2. Message Sending Overview
The act of sending an email message can be thought of as involving
multiple steps: initiation of a new draft, draft editing, message
assembly, and message submission.
Initiation of a new draft and draft editing takes place in the MUA.
Frequently, users choose to save more complex messages on an [IMAP]
server (via the APPEND command with the \Draft flag) for later recall
by the MUA and resumption of the editing process.
Message assembly is the process of producing a complete message from
the final revision of the draft and external sources. At assembly
time, external data is retrieved and inserted in the message.
Message submission is the process of inserting the assembled message
into the [ESMTP] infrastructure, typically using the [SUBMIT]
protocol.
Cridland, et al. Standards Track [Page 16]
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8.3. Traditional Strategy
Traditionally, messages are initiated, edited, and assembled entirely
within an MUA, although drafts may be saved to an [IMAP] server and
later retrieved from the server. The completed text is then
transmitted to a Message Submission Agent (MSA) for delivery.
There is often no clear boundary between the editing and assembly
processes. If a message is forwarded, its content is often retrieved
immediately and inserted into the message text. Similarly, when
external content is inserted or attached, the content is usually
retrieved immediately and made part of the draft.
As a consequence, each save of a draft and subsequent retrieval of
the draft transmits that entire (possibly large) content, as does
message submission.
In the past, this was not much of a problem, because drafts, external
data, and the message submission mechanism were typically located on
the same system as the MUA. The most common problem was running out
of disk quota.
Cridland, et al. Standards Track [Page 17]
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8.4. A New Strategy
The model distinguishes between a Message User Agent (MUA), an
IMAPv4Rev1 Server ([IMAP]), and an SMTP submit server ([SUBMIT]), as
illustrated in Figure 1.
+--------------------+ +--------------+
| | <------------ | |
| MUA (M) | | IMAPv4Rev1 |
| | | Server |
| | ------------> | (Server I) |
+--------------------+ +--------------+
^ | ^ |
| | | |
| | | |
| | | |
| | | |
| | | |
| | | v
| | +--------------+
| |----------------------> | SMTP |
| | Submit |
|-----------------------------| Server |
| (Server S) |
+--------------+
Figure 1: Lemonade "forward without download"
Lemonade "forward without download" allows a Message User Agent to
compose and forward an email combining fragments that are located in
an IMAP server, without having to download these fragments to the
client.
This section informatively describes two ways to perform "forward
without download" based on where the message assembly takes place.
The first uses the extended APPEND command [IMAP-CATENATE] to edit a
draft message in the message store and cause the message assembly on
the IMAP server. This is most often used when a copy of the message
is to be retained on the IMAP server, as discussed in Section 8.6.
The second uses a succession of BURL and BDAT commands to submit and
assemble through concatenation, message data from the client and
external data fetched from the provided URL. The two subsequent
sections provide step-by-step instructions on how "forward without
download" is achieved.
Cridland, et al. Standards Track [Page 18]
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8.4.1. Message Assembly Using IMAP CATENATE Extension
In the [SMTP-BURL]/[IMAP-CATENATE] variant of the Lemonade "forward
without download" strategy, messages are initially composed and
edited within an MUA. The [IMAP-CATENATE] extension to [IMAP] is
then used to create the messages on the IMAP server by transmitting
new text and assembling them. The UIDPLUS [IMAP-UIDPLUS] IMAP
extension is used by the client in order to learn the UID of the
created messages. Finally, an [IMAP-URLAUTH] format URL is given to
a [SUBMIT] server for submission using the BURL [SMTP-BURL]
extension.
The flow involved to support such a use case consists of:
M: {to I -- Optional} The client connects to the IMAP server,
optionally starts TLS (if data confidentiality is required),
authenticates, opens a mailbox ("INBOX" in the example below), and
fetches body structures (see [IMAP]).
Example:
M: A0051 UID FETCH 25627 (UID BODYSTRUCTURE)
I: * 161 FETCH (UID 25627 BODYSTRUCTURE (("TEXT" "PLAIN"
("CHARSET" "US-ASCII") NIL NIL "7BIT" 1152 23)(
"TEXT" "PLAIN" ("CHARSET" "US-ASCII" "NAME"
"trip.txt")
"<960723163407.20117h@washington.example.com>"
"Your trip details" "BASE64" 4554 73) "MIXED"))
I: A0051 OK completed
M: {to I} The client invokes CATENATE (see [IMAP-CATENATE] for
details of the semantics and steps) -- this allows the MUA to create
messages on the IMAP server using new data combined with one or more
message parts already present on the IMAP server.
Note that the example for this step doesn't use the LITERAL+
[IMAP-LITERAL+] extension. Without LITERAL+ the new message is
constructed using three round trips. If LITERAL+ is used, the new
message can be constructed using one round trip.
Cridland, et al. Standards Track [Page 19]
RFC 5550 Lemonade Profile August 2009
M: A0052 APPEND Sent FLAGS (\Draft \Seen $MDNSent)
CATENATE (TEXT {475}
I: + Ready for literal data
M: Message-ID: <419399E1.6000505@caernarfon.example.org>
M: Date: Thu, 12 Nov 2004 16:57:05 +0000
M: From: Bob Ar <bar@example.org>
M: MIME-Version: 1.0
M: To: foo@example.net
M: Subject: About our holiday trip
M: Content-Type: multipart/mixed;
M: boundary="------------030308070208000400050907"
M:
M: --------------030308070208000400050907
M: Content-Type: text/plain; format=flowed
M:
M: Our travel agent has sent the updated schedule.
M:
M: Cheers,
M: Bob
M: --------------030308070208000400050907
M: URL "/INBOX;UIDVALIDITY=385759045/;
UID=25627/;Section=2.MIME" URL "/INBOX;
UIDVALIDITY=385759045/;UID=25627/;Section=2" TEXT {44}
I: + Ready for literal data
M:
M: --------------030308070208000400050907--
M: )
I: A0052 OK [APPENDUID 387899045 45] CATENATE Completed
M: {to I} The client uses the GENURLAUTH command to request a URLAUTH
URL (see [IMAP-URLAUTH]).
I: {to M} The IMAP server returns a URLAUTH URL suitable for later
retrieval with URLFETCH (see [IMAP-URLAUTH] for details of the
semantics and steps).
M: A0053 GENURLAUTH "imap://bob.ar@example.org/Sent;
UIDVALIDITY=387899045/;uid=45;expire=2005-10-
28T23:59:59Z;urlauth=submit+bob.ar" INTERNAL
I: * GENURLAUTH "imap://bob.ar@example.org/Sent;
UIDVALIDITY=387899045/;uid=45;expire=
2005-10-28T23:59:59Z;urlauth=submit+bob.ar:
internal:91354a473744909de610943775f92038"
I: A0053 OK GENURLAUTH completed
M: {to S} The client connects to the mail Submission Server and
starts a new mail transaction. It uses BURL to let the SMTP submit
server fetch the content of the message from the IMAP server (see
[IMAP-URLAUTH] for details of the semantics and steps -- this allows
Cridland, et al. Standards Track [Page 20]
RFC 5550 Lemonade Profile August 2009
the MUA to authorize the SMTP submit server to access the message
composed as a result of the CATENATE step). Note that the second
EHLO command is required after a successful STARTTLS command. Also
note that there might be a third required EHLO command if the second
EHLO response doesn't list any BURL options. Section 8.4.2
demonstrates this.
S: 220 owlry.example.org ESMTP
M: EHLO potter.example.org
S: 250-owlry.example.com
S: 250-8BITMIME
S: 250-BINARYMIME
S: 250-PIPELINING
S: 250-BURL imap
S: 250-CHUNKING
S: 250-AUTH PLAIN
S: 250-DSN
S: 250-SIZE 10240000
S: 250-STARTTLS
S: 250 ENHANCEDSTATUSCODES
M: STARTTLS
S: 220 Ready to start TLS
...TLS negotiation, subsequent data is encrypted...
M: EHLO potter.example.org
S: 250-owlry.example.com
S: 250-8BITMIME
S: 250-BINARYMIME
S: 250-PIPELINING
S: 250-BURL imap
S: 250-CHUNKING
S: 250-AUTH PLAIN
S: 250-DSN
S: 250-SIZE 10240000
S: 250 ENHANCEDSTATUSCODES
M: AUTH PLAIN aGFycnkAaGFycnkAYWNjaW8=
M: MAIL FROM:<bob.ar@example.org>
M: RCPT TO:<foo@example.net>
S: 235 2.7.0 PLAIN authentication successful.
S: 250 2.5.0 Address Ok.
S: 250 2.1.5 foo@example.net OK.
M: BURL imap://bob.ar@example.org/Sent;UIDVALIDITY=387899045/;
uid=45/;urlauth=submit+bar:internal:
91354a473744909de610943775f92038 LAST
S: {to I} The mail Submission Server uses URLFETCH to fetch the
message to be sent. (See [IMAP-URLAUTH] for details of the semantics
and steps. The so-called "pawn-ticket" authorization mechanism uses
a URI that contains its own authorization credentials.)
Cridland, et al. Standards Track [Page 21]
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I: {to S} Provides the message composed as a result of the CATENATE
step).
The mail Submission Server opens an IMAP connection to the IMAP
server:
I: * OK [CAPABILITY IMAP4REV1 STARTTLS NAMESPACE LITERAL+
CATENATE URLAUTH UIDPLUS CONDSTORE IDLE] imap.example.com
IMAP server ready
S: a000 STARTTLS
I: a000 Start TLS negotiation now
...TLS negotiation, if successful - subsequent data
is encrypted...
S: a001 LOGIN submitserver secret
I: a001 OK submitserver logged in
S: a002 URLFETCH "imap://bob.ar@example.org/Sent;
UIDVALIDITY=387899045/;uid=45/;urlauth=submit+bob.ar:
internal:91354a473744909de610943775f92038"
I: * URLFETCH "imap://bob.ar@example.org/Sent;
UIDVALIDITY=387899045/;uid=45/;urlauth=submit+bob.ar:
internal:91354a473744909de610943775f92038" {15065}
...message body follows...
I: a002 OK URLFETCH completed
S: a003 LOGOUT
I: * BYE See you later
I: a003 OK Logout successful
Note that if data confidentiality is not required, the mail
Submission Server may omit the STARTTLS command before issuing the
LOGIN command.
S: {to M} Submission server assembles the complete message; if the
assembly succeeds, it returns OK to the MUA:
S: 250 2.5.0 Ok.
M: {to I} The client marks the message containing the forwarded
attachment on the IMAP server.
M: A0054 UID STORE 25627 +FLAGS.SILENT ($Forwarded)
I: * 215 FETCH (UID 25627 MODSEQ (12121231000))
I: A0054 OK STORE completed
Note: the UID STORE command shown above will only work if the marked
message is in the currently selected mailbox; otherwise, it requires
a SELECT. This command can be omitted, as it simply changes non-
operational metadata not essential to client operations or
Cridland, et al. Standards Track [Page 22]
RFC 5550 Lemonade Profile August 2009
interoperability. The untagged FETCH response is due to
[IMAP-CONDSTORE]. The $Forwarded IMAP keyword is described in
Section 5.9.
8.4.2. Message Assembly Using SMTP CHUNKING and BURL Extensions
In the [IMAP-URLAUTH]/[SMTP-BURL] variant of the Lemonade "forward
without download" strategy, messages are initially composed and
edited within an MUA. During submission [SUBMIT], BURL [SMTP-BURL]
and BDAT [SMTP-BINARYMIME] commands are used to create the messages
from multiple parts. New body parts are supplied using BDAT
commands, while existing body parts are referenced using
[IMAP-URLAUTH] format URLs in BURL commands.
The flow involved to support such a use case consists of:
M: {to I -- Optional} The client connects to the IMAP server,
optionally starts TLS (if data confidentiality is required),
authenticates, opens a mailbox ("INBOX" in the example below), and
fetches body structures (see [IMAP]).
Example:
M: B0051 UID FETCH 25627 (UID BODYSTRUCTURE)
I: * 161 FETCH (UID 25627 BODYSTRUCTURE (("TEXT" "PLAIN"
("CHARSET" "US-ASCII") NIL NIL "7BIT" 1152 23)(
"TEXT" "PLAIN" ("CHARSET" "US-ASCII" "NAME"
"trip.txt")
"<960723163407.20117h@washington.example.com>"
"Your trip details" "BASE64" 4554 73) "MIXED"))
I: B0051 OK completed
M: {to I} The client uses the GENURLAUTH command to request URLAUTH
URLs (see [IMAP-URLAUTH]) referencing pieces of the message to be
assembled.
I: {to M} The IMAP server returns URLAUTH URLs suitable for later
retrieval with URLFETCH (see [IMAP-URLAUTH] for details of the
semantics and steps).
Cridland, et al. Standards Track [Page 23]
RFC 5550 Lemonade Profile August 2009
M: B0052 GENURLAUTH "imap://bob.ar@example.org/INBOX;
UIDVALIDITY=385759045/;UID=25627/;Section=2.MIME;
expire=2006-10-28T23:59:59Z;urlauth=submit+bob.ar"
INTERNAL "imap://bob.ar@example.org/INBOX;
UIDVALIDITY=385759045/;UID=25627/;Section=2;
expire=2006-10-28T23:59:59Z;urlauth=submit+bob.ar" INTERNAL
I: * GENURLAUTH "imap://bob.ar@example.org/INBOX;
UIDVALIDITY=385759045/;UID=25627/;Section=2.MIME;
expire=2006-10-28T23:59:59Z;urlauth=submit+bob.ar:
internal:A0DEAD473744909de610943775f9BEEF"
"imap://bob.ar@example.org/INBOX;
UIDVALIDITY=385759045/;UID=25627/;Section=2;
expire=2006-10-28T23:59:59Z;urlauth=submit+bob.ar:
internal:BEEFA0DEAD473744909de610943775f9"
I: B0052 OK GENURLAUTH completed
M: {to S} The client connects to the mail Submission Server and
starts a new mail transaction. It uses BURL to instruct the SMTP
submit server to fetch from the IMAP server pieces of the message to
be sent (see [SMTP-BURL] for details of the semantics and steps).
Note that the second EHLO command is required after a successful
STARTTLS command. The third EHLO command is required if and only if
the second EHLO response doesn't list any BURL options. See
Section 8.4.1 for an example of submission where the third EHLO
command/response is not present.
S: 220 owlry.example.org ESMTP
M: EHLO potter.example.org
S: 250-owlry.example.com
S: 250-8BITMIME
S: 250-BINARYMIME
S: 250-PIPELINING
S: 250-BURL
S: 250-CHUNKING
S: 250-AUTH DIGEST-MD5
S: 250-DSN
S: 250-SIZE 10240000
S: 250-STARTTLS
S: 250 ENHANCEDSTATUSCODES
M: STARTTLS
S: 220 Ready to start TLS
...TLS negotiation, subsequent data is encrypted...
M: EHLO potter.example.org
S: 250-owlry.example.com
S: 250-8BITMIME
S: 250-BINARYMIME
S: 250-PIPELINING
Cridland, et al. Standards Track [Page 24]
RFC 5550 Lemonade Profile August 2009
S: 250-BURL
S: 250-CHUNKING
S: 250-AUTH DIGEST-MD5 CRAM-MD5 PLAIN EXTERNAL
S: 250-DSN
S: 250-SIZE 10240000
S: 250 ENHANCEDSTATUSCODES
M: AUTH PLAIN aGFycnkAaGFycnkAYWNjaW8=
S: 235 2.7.0 PLAIN authentication successful.
M: EHLO potter.example.org
S: 250-owlry.example.com
S: 250-8BITMIME
S: 250-BINARYMIME
S: 250-PIPELINING
S: 250-BURL imap imap://imap.example.org
S: 250-CHUNKING
S: 250-AUTH DIGEST-MD5 CRAM-MD5 PLAIN EXTERNAL
S: 250-DSN
S: 250-SIZE 10240000
S: 250 ENHANCEDSTATUSCODES
M: MAIL FROM:<bob.ar@example.org> BODY=BINARY
S: 250 2.5.0 Address Ok.
M: RCPT TO:<foo@example.net>
S: 250 2.1.5 foo@example.net OK.
M: BDAT 475
M: Message-ID: <419399E1.6000505@caernarfon.example.org>
M: Date: Thu, 12 Nov 2004 16:57:05 +0000
M: From: Bob Ar <bar@example.org>
M: MIME-Version: 1.0
M: To: foo@example.net
M: Subject: About our holiday trip
M: Content-Type: multipart/mixed;
M: boundary="------------030308070208000400050907"
M:
M: --------------030308070208000400050907
M: Content-Type: text/plain; format=flowed
M:
M: Our travel agent has sent the updated schedule.
M:
M: Cheers,
M: Bob
M: --------------030308070208000400050907
S: 250 2.5.0 OK
M: BURL imap://bob.ar@example.org/INBOX;
UIDVALIDITY=385759045/;UID=25627/;Section=2.MIME;
expire=2006-10-28T23:59:59Z;urlauth=submit+bob.ar:
internal:A0DEAD473744909de610943775f9BEEF
S: 250 2.5.0 OK
M: BURL imap://bob.ar@example.org/INBOX;
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UIDVALIDITY=385759045/;UID=25627/;Section=2;
expire=2006-10-28T23:59:59Z;urlauth=submit+bob.ar:
internal:BEEFA0DEAD473744909de610943775f9
S: 250 2.5.0 OK
M: BDAT 44 LAST
M:
M: --------------030308070208000400050907--
S: {to I} The mail Submission Server uses URLFETCH to fetch the
pieces of the message to be sent. (See [SMTP-BURL] for details of
the semantics and steps. The so-called "pawn-ticket" authorization
mechanism uses a URI which contains its own authorization
credentials.).
I: {to S} Returns the requested body parts.
The mail Submission Server opens an IMAP connection to the IMAP
server:
I: * OK [CAPABILITY IMAP4REV1 STARTTLS NAMESPACE LITERAL+
CATENATE URLAUTH UIDPLUS CONDSTORE IDLE] imap.example.com
IMAP server ready
S: b000 STARTTLS
I: b000 Start TLS negotiation now
...TLS negotiation, if successful - subsequent data
is encrypted...
S: b001 LOGIN submitserver secret
I: b001 OK submitserver logged in
S: b002 URLFETCH "imap://bob.ar@example.org/INBOX;
UIDVALIDITY=385759045/;UID=25627/;Section=2.MIME;
expire=2006-10-28T23:59:59Z;urlauth=submit+bob.ar:
internal:A0DEAD473744909de610943775f9BEEF" "imap://
bob.ar@example.org/INBOX;
UIDVALIDITY=385759045/;UID=25627/;Section=2;
expire=2006-10-28T23:59:59Z;urlauth=submit+bob.ar:
internal:BEEFA0DEAD473744909de610943775f9"
I: * URLFETCH "imap://bob.ar@example.org/INBOX;
UIDVALIDITY=385759045/;UID=25627/;Section=2.MIME;
expire=2006-10-28T23:59:59Z;urlauth=submit+bob.ar:
internal:A0DEAD473744909de610943775f9BEEF" {84}
...message section follows...
"imap://bob.ar@example.org/INBOX;
UIDVALIDITY=385759045/;UID=25627/;Section=2;
expire=2006-10-28T23:59:59Z;urlauth=submit+bob.ar:
internal:BEEFA0DEAD473744909de610943775f9" {15065}
...message section follows...
I: b002 OK URLFETCH completed
S: b003 LOGOUT
I: * BYE See you later
Cridland, et al. Standards Track [Page 26]
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I: b003 OK Logout successful
Note that if data confidentiality is not required, the mail
Submission Server may omit the STARTTLS command before issuing the
LOGIN command.
S: {to M} Submission Server assembles the complete message; if the
assembly succeeds, it acknowledges acceptance of the message by
sending 250 response to the last BDAT command:
S: 250 2.5.0 Ok, message accepted.
M: {to I} The client marks the message containing the forwarded
attachment on the IMAP server.
M: B0053 UID STORE 25627 +FLAGS.SILENT ($Forwarded)
I: * 215 FETCH (UID 25627 MODSEQ (12121231000))
I: B0053 OK STORE completed
Note: the UID STORE command shown above will only work if the marked
message is in the currently selected mailbox; otherwise, it requires
a SELECT. As in the previous example, this command is not critical,
and can be omitted. The untagged FETCH response is due to
[IMAP-CONDSTORE]. The $Forwarded IMAP keyword is described in
Section 5.9.
8.5. Security Considerations for Pawn-Tickets
The so-called "pawn-ticket" authorization mechanism uses a URI, which
contains its own authorization credentials using [IMAP-URLAUTH]. The
advantage of this mechanism is that the SMTP submit [SUBMIT] server
cannot access any data on the [IMAP-URLAUTH] server without a "pawn-
ticket" created by the client.
The "pawn-ticket" grants access only to the specific data that the
SMTP submit [SUBMIT] server is authorized to access, can be revoked
by the client, and can have a time-limited validity.
8.6. Copies of Sent Messages: The fcc Problem
The "fcc problem" refers to delivering a copy of a message to a
mailbox, or "file carbon copy". By far, the most common case of fcc
is a client leaving a copy of outgoing mail in a "Sent Mail" or
"Outbox" mailbox.
In the traditional strategy, the MUA duplicates the effort spent in
transmitting to the MSA by writing the message to the fcc destination
in a separate step. This may be a write to a local disk file or an
Cridland, et al. Standards Track [Page 27]
RFC 5550 Lemonade Profile August 2009
APPEND to a mailbox on an IMAP server. The latter is one of the
"repetitive network data transmissions" that represents the "problem"
aspect of the "fcc problem".
The BURL [SMTP-BURL] extension can be used to eliminate the
additional transmission. The final message is uploaded to the
mailbox designed for outgoing mail by the APPEND command of [IMAP].
Note that when doing so, the client ought to use the $SubmitPending
and $Submitted IMAP keywords described in Section 5.10. Also note
that APPEND, including when enhanced by [IMAP-CATENATE], can only
create a single copy of the message and this is only of use on the
server that stages the outgoing message for submission. Additional
copies of the message on the same server can be created by using one
or more COPY commands.
9. Deployment Considerations
Deployment considerations are discussed extensively in
[LEMONADE-DEPLOYMENTS].
10. Security Considerations
Implementors are advised to examine the security considerations of
all the referenced documents. This section merely highlights these,
and advises implementors on specific issues relating to the
combination of extensions.
Security considerations on Lemonade "forward without download" are
discussed throughout Section 8. Additional security considerations
can be found in [IMAP], [SUBMIT], [SIEVE], and other documents
describing other SMTP, IMAP, and Sieve extension comprising the
Lemonade Profile.
Note that the mandatory-to-implement authentication mechanism for
SMTP submission is described in [SMTP-AUTH]. The mandatory-to-
implement authentication mechanism for IMAP is described in [IMAP].
10.1. Confidentiality Protection of Submitted Messages
When clients submit new messages, link protection such as [TLS]
guards against an eavesdropper seeing the contents of the submitted
message. It is worth noting, however, that even if TLS is not used,
the security risks are no worse if BURL is used to reference the text
than if the text is submitted directly. If BURL is not used, an
eavesdropper gains access to the full text of the message. If BURL
is used, the eavesdropper may or may not be able to gain such access,
Cridland, et al. Standards Track [Page 28]
RFC 5550 Lemonade Profile August 2009
depending on the form of BURL used. For example, some forms restrict
use of the URL to an entity authorized as a Submission Server or a
specific user.
10.2. TLS
When Lemonade clients use the BURL extension for mail submission, an
extension that requires sending a URLAUTH token to the mail
Submission Server, such a token should be protected from interception
to avoid a replay attack that may disclose the contents of the
message to an attacker. [TLS]-based encryption of both the IMAP
session that issues GENURLAUTH and the mail submission path will
provide protection against this attack.
Lemonade-compliant mail Submission Servers SHOULD use TLS-protected
IMAP connections when fetching message content using the URLAUTH
token provided by the Lemonade client.
When a client uses SMTP STARTTLS to send a BURL command that
references non-public information, there is a user expectation that
the entire message content will be treated confidentially. To meet
this expectation, the message Submission Server SHOULD use STARTTLS
or a mechanism providing equivalent data confidentiality when
fetching the content referenced by that URL.
10.3. Additional Extensions and Deployment Models
This specification provides no additional security measures beyond
those in the referenced Internet Mail and Lemonade documents.
We note, however, the security risks associated with:
o Outband notifications
o Server configuration by client
o Client configuration by server
o Presence of proxy servers
o Presence of servers as intermediaries
o In general, the deployment models considered by OMA MEM that are
not conventional IETF deployment models
o Measures to address a perceived need to traverse firewalls and
mobile network intermediaries
Cridland, et al. Standards Track [Page 29]
RFC 5550 Lemonade Profile August 2009
Deployments that provide these additional services or operate in
these environments need to consult the security considerations for
the relevant standards and organizational security practices.
11. IANA Considerations
IMAP4 capabilities are registered by IETF Review, as defined in
[RFC5226]. This document defines the URL-PARTIAL IMAP capability
(Section 5.7.1). IANA added this extension to the IANA IMAP
Capability registry.
12. Changes since RFC 4550
When compared to RFC 4550, this document adds the following
additional requirements on a Lemonade compliant IMAP server:
IMAP extensions: BINARY, COMPRESS=DEFLATE, CONTEXT=SEARCH,
CONTEXT=SORT, CONVERT, ENABLE, ESEARCH, ESORT, I18NLEVEL=1,
NOTIFY, QRESYNC, SASL-IR, SORT, URL-PARTIAL;
IMAP keywords: $SubmitPending, $Submitted.
Other requirements: Require any Lemonade compliant IMAP server to
support the CAPABILITY response code.
When compared to RFC 4550, this document adds the following new
requirements on a Lemonade compliant Message Delivery Agents:
Support for the Sieve filtering language, together with the following
Sieve extensions:
ENOTIFY, IMAP4FLAGS, RELATIONAL, VACATION, VARIABLES, comparator-
i;unicode-casemap.
When compared to RFC 4550, this document recommends use of the
DEFLATE compression method for TLS. All other requirements remain
the same.
Additionally, the following changes/improvments were done to RFC 4550
(the list might be incomplete):
A new section with some additional requirements on Lemonade Mail
User Agents was added, in particular they are required to support
Format=flowed parameter to the text/plain media type.
Usage of the $Forwarded IMAP keyword was clarified.
Forward-without-download examples were corrected and extended.
Cridland, et al. Standards Track [Page 30]
RFC 5550 Lemonade Profile August 2009
Added a new section describing in-band and out-of-band
notifications from a Lemonade compliant mailstore.
13. Acknowledgements
The editors acknowledge and appreciate the work and comments of the
IETF Lemonade working group and the OMA MEM working group.
In particular, the editors would like to thank Eric Burger, Glenn
Parsons, Randall Gellens, Filip Navara, Zoltan Ordogh, Greg
Vaudreuil, and Fan Xiaohui for their comments and reviews.
14. References
14.1. Normative References
[FLOWED] Gellens, R., "The Text/Plain Format and DelSp Parameters",
RFC 3676, February 2004.
[IMAP] Crispin, M., "INTERNET MESSAGE ACCESS PROTOCOL - VERSION
4rev1", RFC 3501, March 2003.
[IMAP-BINARY]
Nerenberg, L., "IMAP4 Binary Content Extension", RFC 3516,
April 2003.
[IMAP-CATENATE]
Resnick, P., "Internet Message Access Protocol (IMAP)
CATENATE Extension", RFC 4469, April 2006.
[IMAP-COMPRESS]
Gulbrandsen, A., "The IMAP COMPRESS Extension", RFC 4978,
August 2007.
[IMAP-CONDSTORE]
Melnikov, A. and S. Hole, "IMAP Extension for Conditional
STORE Operation or Quick Flag Changes Resynchronization",
RFC 4551, June 2006.
[IMAP-CONTEXT]
Cridland, D. and C. King, "Contexts for IMAP4", RFC 5267,
July 2008.
[IMAP-CONVERT]
Melnikov, A. and P. Coates, "Internet Message Access
Protocol - CONVERT Extension", RFC 5259, July 2008.
Cridland, et al. Standards Track [Page 31]
RFC 5550 Lemonade Profile August 2009
[IMAP-ENABLE]
Gulbrandsen, A. and A. Melnikov, "The IMAP ENABLE
Extension", RFC 5161, March 2008.
[IMAP-ESEARCH]
Melnikov, A. and D. Cridland, "IMAP4 Extension to SEARCH
Command for Controlling What Kind of Information Is
Returned", RFC 4731, November 2006.
[IMAP-I18N]
Newman, C., Gulbrandsen, A., and A. Melnikov, "Internet
Message Access Protocol Internationalization", RFC 5255,
June 2008.
[IMAP-IDLE]
Leiba, B., "IMAP4 IDLE command", RFC 2177, June 1997.
[IMAP-LITERAL+]
Myers, J., "IMAP4 non-synchronizing literals", RFC 2088,
January 1997.
[IMAP-NAMESPACE]
Gahrns, M. and C. Newman, "IMAP4 Namespace", RFC 2342,
May 1998.
[IMAP-NOTIFY]
Gulbrandsen, A., King, C., and A. Melnikov, "The IMAP
NOTIFY Extension", RFC 5465, February 2009.
[IMAP-QRESYNC]
Melnikov, A., Cridland, D., and C. Wilson, "IMAP4
Extensions for Quick Mailbox Resynchronization", RFC 5162,
March 2008.
[IMAP-SASL-IR]
Siemborski, R. and A. Gulbrandsen, "IMAP Extension for
Simple Authentication and Security Layer (SASL) Initial
Client Response", RFC 4959, September 2007.
[IMAP-SORT]
Crispin, M. and K. Murchison, "Internet Message Access
Protocol - SORT and THREAD Extensions", RFC 5256,
June 2008.
[IMAP-UIDPLUS]
Crispin, M., "Internet Message Access Protocol (IMAP) -
UIDPLUS extension", RFC 4315, December 2005.
Cridland, et al. Standards Track [Page 32]
RFC 5550 Lemonade Profile August 2009
[IMAP-URL]
Melnikov, A. and C. Newman, "IMAP URL Scheme", RFC 5092,
November 2007.
[IMAP-URLAUTH]
Crispin, M., "Internet Message Access Protocol (IMAP) -
URLAUTH Extension", RFC 4467, May 2006.
[KEYWORDS]
Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[SIEVE] Guenther, P. and T. Showalter, "Sieve: An Email Filtering
Language", RFC 5228, January 2008.
[SIEVE-IMAP4FLAGS]
Melnikov, A., "Sieve Email Filtering: Imap4flags
Extension", RFC 5232, January 2008.
[SIEVE-NOTIFY]
Melnikov, A., Leiba, B., Segmuller, W., and T. Martin,
"Sieve Email Filtering: Extension for Notifications",
RFC 5435, January 2009.
[SIEVE-RELATIONAL]
Segmuller, W. and B. Leiba, "Sieve Email Filtering:
Relational Extension", RFC 5231, January 2008.
[SIEVE-VACATION]
Showalter, T. and N. Freed, "Sieve Email Filtering:
Vacation Extension", RFC 5230, January 2008.
[SIEVE-VARIABLES]
Homme, K., "Sieve Email Filtering: Variables Extension",
RFC 5229, January 2008.
[SMTP-8BITMIME]
Klensin, J., Freed, N., Rose, M., Stefferud, E., and D.
Crocker, "SMTP Service Extension for 8bit-MIMEtransport",
RFC 1652, July 1994.
[SMTP-AUTH]
Siemborski, R. and A. Melnikov, "SMTP Service Extension
for Authentication", RFC 4954, July 2007.
Cridland, et al. Standards Track [Page 33]
RFC 5550 Lemonade Profile August 2009
[SMTP-BINARYMIME]
Vaudreuil, G., "SMTP Service Extensions for Transmission
of Large and Binary MIME Messages", RFC 3030,
December 2000.
[SMTP-BURL]
Newman, C., "Message Submission BURL Extension", RFC 4468,
May 2006.
[SMTP-DSN]
Moore, K., "Simple Mail Transfer Protocol (SMTP) Service
Extension for Delivery Status Notifications (DSNs)",
RFC 3461, January 2003.
[SMTP-PIPELINING]
Freed, N., "SMTP Service Extension for Command
Pipelining", STD 60, RFC 2920, September 2000.
[SMTP-SIZE]
Klensin, J., Freed, N., and K. Moore, "SMTP Service
Extension for Message Size Declaration", STD 10, RFC 1870,
November 1995.
[SMTP-STATUSCODES]
Freed, N., "SMTP Service Extension for Returning Enhanced
Error Codes", RFC 2034, October 1996.
[SMTP-TLS]
Hoffman, P., "SMTP Service Extension for Secure SMTP over
the Transport Layer Security", RFC 3207, February 2002.
[SUBMIT] Gellens, R. and J. Klensin, "Message Submission for Mail",
RFC 4409, April 2006.
[TLS] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246, August 2008.
[TLS-COMP]
Hollenbeck, S., "Transport Layer Security Protocol
Compression Methods", RFC 3749, May 2004.
[UNICODE-CASEMAP]
Crispin, M., "i;unicode-casemap - Simple Unicode Collation
Algorithm", RFC 5051, October 2007.
Cridland, et al. Standards Track [Page 34]
RFC 5550 Lemonade Profile August 2009
14.2. Informative References
[ESMTP] Klensin, J., "Simple Mail Transfer Protocol", RFC 5321,
October 2008.
[Err1807] RFC Errata, Errata ID 1807, RFC 5162,
<http://www.rfc-editor.org>.
[Err1808] RFC Errata, Errata ID 1808, RFC 5162,
<http://www.rfc-editor.org>.
[Err1809] RFC Errata, Errata ID 1809, RFC 5162,
<http://www.rfc-editor.org>.
[Err1810] RFC Errata, Errata ID 1810, RFC 5162,
<http://www.rfc-editor.org>.
[FINGER-HACK]
Gellens, R., "Simple New Mail Notification", RFC 4146,
August 2005.
[IMAP-FILTERS]
Melnikov, A. and C. King, "IMAP4 Extension for Named
Searches (Filters)", RFC 5466, February 2009.
[IMAP-SYNC-HOWTO]
Melnikov, A., "Synchronization Operations for Disconnected
IMAP4 Clients", RFC 4549, June 2006.
[LEMONADE-ARCH]
Burger, E. and G. Parsons, "LEMONADE Architecture -
Supporting Open Mobile Alliance (OMA) Mobile Email (MEM)
Using Internet Mail", RFC 5442, March 2009.
[LEMONADE-DEPLOYMENTS]
Gellens, R., "Deployment Considerations for Lemonade-
Compliant Mobile Email", BCP 143, RFC 5383, October 2008.
[LEMONADE-NOTIFICATIONS]
Gellens, R., Ed., "Lemonade Notifications Architecture",
RFC 5551, August 2009.
[MANAGESIEVE]
Melnikov, A. and T. Martin, "A Protocol for Remotely
Managing Sieve Scripts", Work in Progress, September 2008.
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RFC 5550 Lemonade Profile August 2009
[METADATA]
Daboo, C., "The IMAP METADATA Extension", RFC 5464,
February 2009.
[OMA-EMN] Open Mobile Alliance, "Open Mobile Alliance Email
Notification Version 1.0", OMA http://
www.openmobilealliance.org/Technical/release_program/
emn_v10.aspx, October 2007.
[OMA-MEM-ARCH]
Open Mobile Alliance, "Mobile Email Architecture
Document", OMA (Work in Progress),
http://www.openmobilealliance.org/, October 2005.
[OMA-MEM-REQ]
Open Mobile Alliance, "Mobile Email Requirements
Document", OMA http://www.openmobilealliance.org/
release_program/docs/RD/
OMA-RD-MobileEmail-V1_0_20051018-C.pdf, Oct 2005.
[RFC5068] Hutzler, C., Crocker, D., Resnick, P., Allman, E., and T.
Finch, "Email Submission Operations: Access and
Accountability Requirements", BCP 134, RFC 5068,
November 2007.
[RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", BCP 26, RFC 5226,
May 2008.
[RFC5598] Crocker, D., "Internet Mail Architecture", RFC 5598,
July 2009.
Cridland, et al. Standards Track [Page 36]
RFC 5550 Lemonade Profile August 2009
Appendix A. Errata
Errata ID: 1807 [Err1807]
Status: Verified
Type: Technical
Reported By: Timo Sirainen
Date Reported: 2009-07-14
Verifier Name: Alexey Melnikov
Date Verified: 2009-07-18
Section 1 says:
It should say:
Once a "CONDSTORE enabling command" is issued by the client, the
server MUST automatically include both UID and mod-sequence data in
all subsequent untagged FETCH responses (until the connection is
closed), whether they were caused by a regular STORE/UID STORE, a
STORE/UID STORE with UNCHANGEDSINCE modifier, or an external agent.
Note that this rule doesn't affect untagged FETCH responses caused by
a FETCH command that doesn't include UID and/or MODSEQ FETCH data
item, or UID FETCH without the MODSEQ FETCH data item.
Notes:
Rationale:
It's very difficult for clients to make use of unsolicited FETCH
responses without the UID field. This is made even worse by the text
that says "servers SHOULD NOT send UIDs for previously expunged
messages [in VANISHED replies]". Since it's not a MUST NOT, a
conversation with an RFC compliant server could be for example:
A1 NOOP
* 0 EXISTS
A1 OK
A2 NOOP
* 10 EXISTS
* VANISHED 1000:2000
* 3 FETCH (FLAGS (\Seen) MODSEQ (14749))
* 5 FETCH (FLAGS (\Seen) MODSEQ (14749))
* VANISHED 2000:3000
A2 OK NOOP Completed
The client couldn't do anything with the information from FETCH
replies, because it can't know what messages they refer to.
Cridland, et al. Standards Track [Page 37]
RFC 5550 Lemonade Profile August 2009
Errata ID: 1808 [Err1808]
Status: Verified
Type: Technical
Reported By: Timo Sirainen
Date Reported: 2009-07-14
Verifier Name: Alexey Melnikov
Date Verified: 2009-07-18
Section 3.4 says:
If at least one message got expunged, the server MUST send
the updated per-mailbox modification
sequence using the HIGHESTMODSEQ response code (defined in
[CONDSTORE]) in the tagged OK response.
Example: C: A202 CLOSE
S: A202 OK [HIGHESTMODSEQ 20010715194045319] done
It should say:
The server MUST NOT send the updated per-mailbox modification
sequence using the HIGHESTMODSEQ response code (defined in
[CONDSTORE]) in the tagged OK response, as this might cause loss of
synchronization on the client.
Example: C: A202 CLOSE
S: A202 OK done
Notes:
Rationale:
The HIGHESTMODSEQ can't be used reliably unless server sends to client
all changes done by other clients. Even then it's difficult for both
clients and servers to implement this. For example:
C1: 2 STORE 1 +FLAGS.SILENT \Deleted
S1: * 1 FETCH (MODSEQ 1)
S1: 2 OK
C2: 1 STORE 2 +FLAGS.SILENT \Deleted
S1: * 2 FETCH (MODSEQ 2)
S2: 1 OK
C1: 3 CLOSE
S1: 3 [HIGHESTMODSEQ 3]
Cridland, et al. Standards Track [Page 38]
RFC 5550 Lemonade Profile August 2009
The client probably thought that only message 1 was expunged, so it
doesn't register the second expunge. And it probably never will if it
uses QRESYNC to find out only about new expunges.
And even worse example would be if the second client had also removed
the \Deleted flag from message 1. Then the first client would have
registered wrong message to be expunged.
Errata ID: 1809 [Err1809]
Status: Verified
Type: Technical
Reported By: Timo Sirainen
Date Reported: 2009-07-14
Verifier Name: Alexey Melnikov
Date Verified: 2009-07-18
Section 5 says:
After completing a full synchronization, the client MUST also take
note of any unsolicited MODSEQ FETCH data items received from the
server. Whenever the client receives a tagged response to a command,
it calculates the highest value among all MODSEQ FETCH data items
received since the last tagged response. If this value is bigger
than the client's copy of the HIGHESTMODSEQ value, then the client
MUST use this value as its new HIGHESTMODSEQ value.
Note: It is not safe to update the client's copy of the HIGHESTMODSEQ
value with a MODSEQ FETCH data item value as soon as it is received
because servers are not required to send MODSEQ FETCH data items in
increasing modseqence order. This can lead to the client missing
some changes in case of connectivity loss.
It should say:
After completing a full synchronization, the client MUST also take
note of any unsolicited MODSEQ FETCH data items and HIGHESTMODSEQ
response codes received from the server. Whenever the client receives
a tagged response to a command, it checks the received unsolicited
responses to calculate the new HIGHESTMODSEQ value. If the
HIGHESTMODSEQ response code is received, the client MUST use it even
if it has seen higher mod-sequences. Otherwise, the client calculates
the highest value among all MODSEQ FETCH data items received since the
last tagged response. If this value is bigger than the client's copy
of the HIGHESTMODSEQ value, then the client MUST use this value as its
new HIGHESTMODSEQ value.
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RFC 5550 Lemonade Profile August 2009
Example: C: A1 STORE 1:2 (UNCHANGEDSINCE 96) +FLAGS.SILENT \Seen
S: * 1 FETCH (UID 6 MODSEQ (103))
S: * 2 FETCH (UID 7 MODSEQ (101))
S: * OK [HIGHESTMODSEQ 99] VANISHED reply with
MODSEQ 100 is delayed
S: A1 OK [MODIFIED 3] done
C: A2 STORE 3 +FLAGS.SILENT \Seen
S: * 3 FETCH (UID 8 MODSEQ (104))
S: A2 OK [HIGHESTMODSEQ 99] Still delaying VANISHED
C: A3 NOOP
S: * VANISHED 8
S: A3 OK [HIGHESTMODSEQ 104] done
Note: It is not safe to update the client's copy of the HIGHESTMODSEQ
value with a MODSEQ FETCH data item value as soon as it is received
because servers are not required to send MODSEQ FETCH data items in
increasing modseqence order. Some commands may also delay EXPUNGE
(or VANISHED) replies with smaller mod-sequences. These can lead to
the client missing some changes in case of connectivity loss.
Notes:
Rationale:
Otherwise clients could lose changes in case of connectivity loss.
Errata ID: 1810 [Err1810]
Status: Verified
Type: Technical
Reported By: Timo Sirainen
Date Reported: 2009-07-14
Verifier Name: Alexey Melnikov
Date Verified: 2009-07-18
Section 1 says:
It should say:
Server implementing QRESYNC MUST send untagged events to client in a
way that client doesn't lose any changes in case of connectivity loss.
In particular this means that if server sends MODSEQ FETCH data items
while EXPUNGE (or VANISHED) replies with lower mod-sequences are being
delayed, the server MUST send HIGHESTMODSEQ response code with a lower
value than the EXPUNGE's mod-sequence. See example in section 5.
Cridland, et al. Standards Track [Page 40]
RFC 5550 Lemonade Profile August 2009
Notes:
This is related to the other errata in section 5, which describes what
the client's behavior should be. This describes what the server's
behavior should be. Would have been nice to put them into the same
section, but that probably would require larger changes.
Authors' Addresses
Dave Cridland (editor)
Isode Limited
5 Castle Business Village
36 Station Road
Hampton, Middlesex TW12 2BX
UK
EMail: dave.cridland@isode.com
Alexey Melnikov (editor)
Isode Limited
5 Castle Business Village
36 Station Road
Hampton, Middlesex TW12 2BX
UK
EMail: Alexey.Melnikov@isode.com
Stephane H. Maes (editor)
Oracle
MS 4op634, 500 Oracle Parkway
Redwood Shores, CA 94539
USA
Phone: +1-203-300-7786
EMail: stephane.maes@oracle.com
Cridland, et al. Standards Track [Page 41]