💾 Archived View for gmi.noulin.net › rfc › rfc1203.gmi captured on 2023-06-16 at 22:42:48. Gemini links have been rewritten to link to archived content
⬅️ Previous capture (2023-01-29)
-=-=-=-=-=-=-
Obsoletes:
Keywords: IMAP3
Network Working Group J. Rice
Request for Comments: 1203 Stanford
Obsoletes: RFC 1064 February 1991
INTERACTIVE MAIL ACCESS PROTOCOL - VERSION 3
Status of this Memo
This RFC suggests a method for workstations to access mail
dynamically from a mailbox server ("repository"). This RFC specifies
a standard for the SUMEX-AIM community and an Experimental Protocol
for the Internet community. Discussion and suggestions for
improvement are requested. Please refer to the current edition of
the "IAB Official Protocol Standards" for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Scope
The following document is a modified version of RFC 1064, the
definition of the IMAP2 protocol. This RFC has been written
specifically as a counter proposal to RFC 1176, which itself proposes
modifications to IMAP2. Sadly, RFC 1176 was made without internal
consultation with the IMAP community, so we are in a position of
feeling we have to present a counter proposal to what, if we do not
act, will become a de facto standard. The reasons for this counter
proposal are numerous but fall mostly into the following categories:
- IMAP2 is insufficiently powerful for a number of server/client
interactions which we believe to be important. RFC 1176
negligibly enhances the functionality of IMAP2.
- IMAP2 makes what we believe to be an erroneous definition for
unsolicited vs. solicited data. IMAP3 as specified herein
attempts to correct this. RFC 1176 makes no effort to remedy
these problems.
- RFC 1176 has explicitly modified the intent of RFC 1064 by
allowing the server to make assumptions about the client's
caching architecture. We believe this to be a grave error
and do not support it in this proposal.
- RFC 1176 specifies a number of "optional" features in the
protocol without specifying a suitable metaprotocol by which
servers and clients can adequately negotiate over the set of
implemented features. This proposal specifies a mechanism
by which servers and clients can come to an unambiguous
understanding about which features are usable by each party.
Rice [Page 1]
RFC 1203 IMAP3 February 1991
- RFC 1176 pays only lip-service to being network protocol
independent and, in fact assumes the use of TCP/IP. Neither
RFC 1064 nor this proposal make any such assumption.
Although there are numerous other detailed objections to RFC 1176, we
believe that the above will serve to show that we believe strongly in
the importance of mailbox abstraction level mail protocols and, after
a couple of years of use of IMAP2 under RFC 1064 we believe that we
have a good enough understanding of the issues involved to be able to
take the next step.
It is important to take this next step because of the rapid pace of
both mail system and user interface development. We believe that,
for IMAP not to die in its infancy, IMAP must be ready to respond to
emerging ISO and RFC standards in mail, such as for multi-media mail.
We believe that RFC 1176 not only provides a very small increment in
functionality over RFC 1064 but also adds a number of bugs, which
would be detrimental to the IMAP cause. Thus we propose the
following definition for IMAP3.
Compatibility notes:
In revising the IMAP2 protocol it has been our intent, wherever
possible to make upwards compatible changes to produce IMAP3. There
were, however, some places that had to be changed incompatibly in
order to compensate for either ambiguities in the IMAP2 protocol as
defined by RFC 1064 or behavior that proved undesirable in the light
of experience.
It is our goal, however, that existing IMAP2 clients should still be
supported and that, at least for the foreseeable future, all IMAP3
servers will support IMAP2 behavior as their default mode.
The following are the major differences between this proposal, RFC
1176 and RFC 1064:
- In this proposal we specify a difference between "solicited" and
"unsolicited" data sent from the server. It is generally the
case that data sent by the server can be sent either in response
to an explicit request by the client or by the server of its own
volition. Any data that the server is required to sent to the
client as the result of a request is said to be solicited and
carries the same tag as the request that provoked it. Any data
sent by the server to the client that is not required by the
protocol is said to be unsolicited and carries the special "*"
tag. RFC 1176 preserves the original RFC 1064 terminology that
calls all such data sent by the server "unsolicited" even when
Rice [Page 2]
RFC 1203 IMAP3 February 1991
it is, in fact, solicited.
- This proposal introduces the experimental concept of
distinguishing between Generic, Canonical and Concrete keys,
allowing the mailbox to be viewed as a relational database
indexed by these keys. This should allow the IMAP protocol
to evolve away from its current reliance on RFC 822. RFC 1176
does not have such a unifying model.
- The SEARCH command has been changed so as to allow multiple
simultaneous searches to be made and to allow unsolicited
search messages to be sent by the server. Such a change is
essential to allow more sophisticated servers that can process
commands asynchronously, possibly substantially delaying
searches over slow backing storage media, for example. It is
also important to allow servers to be able to send unsolicited
search messages that might inform the client of interesting
patterns of messages, such as new and unseen mail.
- This proposal introduces a specific protocol for the negotiation
of protocol versions and server features. This is important
because it allows client/server pairs to come to an agreement on
what behavior is really available to it. RFC 1176 introduces a
number of "optional" commands, which are in some way analogous
to "feature-introduced" commands in this proposal. The principle
distinction between these is that in RFC 1176 there is no way
for a client to discover the set of optional commands, nor is
there a way for it to determine whether a specific command
really is supported, since RFC 1176 requires the use of the
"BAD" response if a feature is not supported. There is,
therefore, no way for the client to determine why the attempted
command did not work. This also means that, for example, a
client cannot disable certain user commands or make them
invisible on menus if they are not supported, since there
is no way for the client to discover whether the commands are
indeed supported without trying to execute such a command.
- This proposal introduces a mechanism for clients to create and
delete user flags (keywords). This is nor supported in either
RFC 1176 or RFC 1064, requiring the user to add keys manually
on the server, generally by editing some form of "init" file.
- RFC 1064 has no mechanism for determining whether a mailbox is
readonly or not. RFC 1176 introduces a non-enforced convention
of encoding data about the readonly status of a mailbox in the
SELECT message's OK respose comment field. This is not regular
with respect to the rest of the protocol, in which the comment
field is used for no purpose other than documentation. This
Rice [Page 3]
RFC 1203 IMAP3 February 1991
proposal introduces specific protocol additions for the dynamic
determination and modification of the readonly/readwrite status
of mailboxes.
Introduction
The intent of the Interactive Mail Access Protocol, Version 3 (IMAP3)
is to allow a (possibly unreliable) workstation or similar machine to
access electronic mail from a reliable mailbox server in an efficient
manner.
Although different in many ways from POP2 (RFC 937), IMAP3 may be
thought of as a functional superset of POP2, and the POP2 RFC was
used as a model for this RFC. There was a cognizant reason for this;
RFC 937 deals with an identical problem and it was desirable to offer
a basis for comparison.
Like POP2, IMAP3 specifies a means of accessing stored mail and not
of posting mail; this function is handled by a mail transfer protocol
such as SMTP (RFC 821). A comparison with the DMSP protocol of
PCMAIL can be found at the end of "System Model and Philosophy"
section.
This protocol assumes a reliable data stream such as provided by TCP
or any similar protocol. When TCP is used, the IMAP server listens
on port 220. When CHAOS is used the IMAP server listens for the
logical contact name "IMAP3".
Communication in IMAP is defined to be using the ASCII character
interpretation of data. Communication using other conventions may be
possible by the selection of features on some servers.
System Model and Philosophy
Electronic mail is a primary means of communication for the widely
spread SUMEX-AIM community. The advent of distributed workstations
is forcing a significant rethinking of the mechanisms employed to
manage such mail. With mainframes, each user tends to receive and
process mail at the computer he used most of the time, his "primary
host". The first inclination of many users when an independent
workstation is placed in front of them is to begin receiving mail at
the workstation, and, in fact, many vendors have implemented
facilities to do this. However, this approach has several
disadvantages:
(1) Workstations (especially Lisp workstations) have a software
design that gives full control of all aspects of the system
to the user at the console. As a result, background tasks,
Rice [Page 4]
RFC 1203 IMAP3 February 1991
like receiving mail, could well be kept from running for
long periods of time either because the user is asking to
use all of the machine's resources, or because, in the course
of working, the user has (perhaps accidentally) manipulated
the environment in such a way as to prevent mail reception.
This could lead to repeated failed delivery attempts by
outside agents.
(2) The hardware failure of a single workstation could keep its
user "off the air" for a considerable time, since repair of
individual workstation units might be delayed. Given the
growing number of workstations spread throughout office
environments, quick repair would not be assured, whereas a
centralized mainframe is generally repaired very soon after
failure.
(3) It is more difficult to keep track of mailing addresses when
each person is associated with a distinct machine. Consider
the difficulty in keeping track of a large number of postal
addresses or phone numbers, particularly if there was no
single address or phone number for an organization through
which you could reach any person in that organization.
Traditionally, electronic mail on the ARPANET involved
remembering a name and one of several "hosts" (machines)
whose name reflected the organization in which the
individual worked. This was suitable at a time when most
organizations had only one central host. It is less
satisfactory today unless the concept of a host is changed
to refer to an organizational entity and not a particular
machine.
(4) It is very difficult to keep a multitude of heterogeneous
workstations working properly with complex mailing protocols,
making it difficult to move forward as progress is made in
electronic communication and as new standards emerge. Each
system has to worry about receiving incoming mail, routing
and delivering outgoing mail, formatting, storing, and
providing for the stability of mailboxes over a variety of
possible filing and mailing protocols.
Consequently, while the workstation may be viewed as an Internet host
in the sense that it implements IP, it should not be viewed as the
entity which contains the user's mailbox. Rather, a mail server
machine (sometimes called a "repository") should hold the mailbox,
and the workstation (hereafter referred to as a "client") should
access the mailbox via mail transactions. Because the mail server
machine would be isolated from direct user manipulation, it could
achieve high software reliability easily, and, as a shared resource,
Rice [Page 5]
RFC 1203 IMAP3 February 1991
it could achieve high hardware reliability, perhaps through
redundancy. The mail server could be used from arbitrary locations,
allowing users to read mail across campus, town, or country using
more and more commonly available clients. Furthermore, the same user
may access his mailbox from different clients at different times, and
multiple users may access the same mailbox simultaneously.
The mail server acts an an interface among users, data storage, and
other mailers. The mail access protocol is used to retrieve
messages, access and change properties of messages, and manage
mailboxes. This differs from some approaches (e.g., Unix mail via
NFS) in that the mail access protocol is used for all message
manipulations, isolating the user and the client from all knowledge
of how the data storage is used. This means that the mail server can
utilize the data storage in whatever way is most efficient to
organize the mail in that particular environment, without having to
worry about storage representation compatibility across different
machines.
In defining a mail access protocol, it is important to keep in mind
that the client and server form a macrosystem, in which it should be
possible to exploit the strong points of both while compensating for
each other's weaknesses. Furthermore, it's desirable to allow for a
growth path beyond the hoary text-only RFC 822 protocol. Unlike
POP2, IMAP3 has extensive features for remote searching and parsing
of messages on the server. For example, a free text search
(optionally in conjunction with other searching) can be made
throughout the entire mailbox by the server and the results made
available to the client without the client having to transfer the
entire mailbox and searching itself. Since remote parsing of a
message into a structured (and standard format) "envelope" is
available, a client can display envelope information and implement
commands such as REPLY without having any understanding of how to
parse RFC 822, etc., headers.
Additionally, IMAP3 offers several facilities for managing a mailbox
beyond the simple "delete message" functionality of POP2.
In spite of this, IMAP3 is a relatively simple protocol. Although
servers should implement the full set of IMAP3 functions, a simple
client can be written which uses IMAP3 in much the way as a POP2
client.
IMAP3 differs from the DMSP protocol of PCMAIL (RFC 1056) in a more
fundamental manner, reflecting the differing architectures of IMAP
and PCMAIL. PCMAIL is either an online ("interactive mode"), or
offline ("batch mode") system. IMAP is primarily an online system in
which real-time and simultaneous mail access were considered
Rice [Page 6]
RFC 1203 IMAP3 February 1991
important.
In PCMAIL, there is a long-term client/server relationship in which
some mailbox state is preserved on the client. There is a
registration of clients used by a particular user, and the client
keeps a set of "descriptors" for each message which summarize the
message. The server and client synchronize their states when the
DMSP connection starts up, and, if a client has not accessed the
server for a while, the client does a complete reset (reload) of its
state from the server.
In IMAP, the client/server relationship lasts only for the duration
of the IMAP3 connection. All mailbox state is maintained on the
server. There is no registration of clients. The function of a
descriptor is handled by a structured representation of the message
"envelope". This structure makes it unnecessary for a client to know
anything about RFC 822 parsing. There is no synchronization since
the client does not remember state between IMAP3 connections. This
is not a problem since in general the client never needs the entire
state of the mailbox in a single session, therefore there isn't much
overhead in fetching the state information that is needed as it is
needed.
There are also some functional differences between IMAP3 and DMSP.
DMSP has functions for sending messages, printing messages, and
changing passwords, all of which are done outside of IMAP3. DMSP has
16 binary flags of which 8 are defined by the system. IMAP has flag
names; there are currently 5 defined system flag names and a facility
for some number (29 in the current implementations) of user flag
names. IMAP3 has a sophisticated message search facility in the
server to identify interesting messages based on dates, addresses,
flag status, or textual contents without compelling the client to
fetch this data for every message.
It was felt that maintaining state on the client is advantageous only
in those cases where the client is only used by a single user, or if
there is some means on the client to restrict access to another
user's data. It can be a serious disadvantage in an environment in
which multiple users routinely use the same client, the same user
routinely uses different clients, and where there are no access
restrictions on the client. It was also observed that most user mail
access is to a relatively small set of "interesting" messages, which
were either "new" mail or mail based upon some user-selected
criteria. Consequently, IMAP3 was designed to easily identify those
"interesting" messages so that the client could fetch the state of
those messages and not those that were not "interesting".
One crucial philosophical difference between IMAP and other common
Rice [Page 7]
RFC 1203 IMAP3 February 1991
mail protocols is that IMAP is a mailbox access protocol, not a
protocol for manipulating mail files. In the IMAP model, unlike
other mail system models in which mail is stored in a linear mail
file, no specification is made for the implementation architecture
for mail storage. Servers may choose to implement mailboxes as files
but this is a detail of which the client can be totally unaware.
What is more, in the IMAP model, mailboxes are viewed as mappings
from keys into values. There are broadly three types of keys,
generic, canonical and concrete. Generic keys are generic, mail
protocol independent keys defined by IMAP which are meaningful across
multiple mail encoding formats. An example of such a generic key
might be "TO", which would be associated with the "To:" field of an
RFC 822 format message.
Canonical keys represent the way in which the server can associate
values that are generally "about" a certain key concept, possibly
integrating several mail format specific fields, without having to
worry the client with the particular details of any particular
message format. Thus, the canonical TO key (called $TO) could denote
anything that could reasonably be construed as being directed towards
someone. Hence, in an RFC 822 message the server could find the
union of the "To:", "Resent-To", "Apparently-To:" and "CC:" fields to
be the appropriate value associated with the canonical $TO key.
Concrete keys allow the client to gain access to certain mail format
specific concepts, that are not pre-specified by the IMAP protocol,
in a well defined manner. For example, If the client asks for the
value associated with the "APPARENTLY-TO" key then, if the message
were to be in RFC 822 format, the server would look for a header
field called "Apparently-To:". If no such field is found or the
field is not implemented or meaningful for the particular message
format then the server will respond with the null value, called NIL,
indicating the non-existence of the field.
Thus, IMAP servers are at liberty to implement mailboxes as a
relational databases if it seems convenient. Indeed, we anticipate
that future mail systems will tend to use database technology for the
storage and indexing of mailboxes as a result of the pressure caused
by the increasing size of mailboxes.
Although for historical reasons IMAP is currently somewhat closely
associated with RFC 822, we anticipate that future developments in
IMAP will remove these mail format specific components and will move
towards the generic model mentioned above. This will allow IMAP more
easily to incorporate such things as multi-media mail.
Rice [Page 8]
RFC 1203 IMAP3 February 1991
The Protocol
The IMAP3 protocol consists of a sequence of client commands and
server responses to those commands, with extra information from the
server data being sent asynchronously to and independent to the
responses to client commands. Unlike most Internet protocols,
commands and responses are tagged. That is, a command begins with a
unique identifier (typically a short alphanumeric sequence such as a
Lisp "gensym" function would generate e.g., A0001, A0002, etc.),
called a tag. The response to this command is given the same tag
from the server.
We distinguish between data sent by the server as the result of a
client request, which we term "SOLICITED" and data sent by the server
not as the result of a client request, which we term "UNSOLICITED".
The server may send unsolicited data at any time that would not
fragment another piece of data on the same stream rendering it
unintelligible. The server is contractually required, however, to
return all data that is solicited by the client before the return of
the completion signal for that command, i.e., all solicited data must
be returned within the temporal extent of the request/completion
acknowledgement wrapper. This does not, however, preclude the
simultaneous processing of multiple requests by the client, it simply
requires that the client be confident that it has all the requested
data when a request finishes. This allows the implementation of both
synchronous and asynchronous clients.
Solicited data is identified by the tag of the initial request by the
client. Unsolicited data is identified by the special reserved tag
of "*". There is another special reserved tag, "+", discussed below.
Note: the tagging of SOLICITED data is only permitted for a selected
server version other than 2.0.
No assumptions concerning serial or monolithic processing by the
server can be made by a correct client. The server is at liberty to
process multiple requests by the same client in any order. This
allows servers to process costly searches over mailboxes on slow
backing storage media in the background, while still preserving
interactive performance. Clients can, however, assume the
serialization of the request/data/completion behavior mentioned
above.
When a connection is opened the server sends an unsolicited OK
response as a greeting message and then waits for commands. When
commands are received the server acts on them and responds with
responses, often interspersed with data.
Rice [Page 9]
RFC 1203 IMAP3 February 1991
The client opens a connection, waits for the greeting, then sends a
LOGIN command with user name and password arguments to establish
authorization. Following an OK response from the server, the client
then sends a SELECT command to access the desired mailbox. The
user's default mailbox has a special reserved name of "INBOX" which
is independent of the operating system that the server is implemented
on. The server will generally send a list of valid flags, number of
messages, and number of messages arrived since last access for this
mailbox as solicited data, followed by an OK response. The client
may terminate access to this mailbox and access a different one with
another SELECT command.
Because the SELECT command affects the state of the server in a
fundamental way, the server is required to process all outstanding
commands for any given mailbox before sending the OK tag for the
SELECT command. Thus, the client will always know that all responses
before an OK SELECT response will refer to the old mailbox and all
responses following it will apply to the new mailbox.
Because, in the real world, local needs or experimental work will
dictate that servers will support both supersets of the defined
behavior and incompatible changes, servers will support a
SELECT.VERSION command and a SELECT.FEATURES command, the purpose of
which is to allow clients to select the overall behavior and specific
features that they want from a server. The default behavior of any
server is to process commands and to have interaction syntax the same
as is specified by IMAP2 in RFC 1064. A server may not behave in any
other manner unless the SELECT.VERSION or SELECT.FEATURES commands
are used to select different behavior.
Over time, when groups of generally useful changes to the current,
default behavior of the server are found, these will be collected
together and incorporated in such a way that all of the features can
be selected simply by selecting a particular major version number of
the protocol. It should be noted that the version numbers (both
major and minor) selected by the SELECT.VERSION command denote
versions of the IMAP protocol, not versions of the server per se.
Thus, although in general changes to the protocol specification will
be made in such a way that they are upwards compatible, this cannot
be guaranteed. No client should rely on tests of the form "if
major_version > 2 then..." being valid for all protocol versions,
since incompatible changes might be made in the future.
The client reads mailbox information by means of FETCH commands. The
actual data is transmitted via the solicited data mechanism (that is,
FETCH should be viewed as poking the server to include the desired
data along with any other data it wishes to transmit to the client).
There are three major categories of data which may be fetched.
Rice [Page 10]
RFC 1203 IMAP3 February 1991
The first category is that data which is associated with a message as
an entity in the mailbox. There are presently three such items of
data: the "internal date", the "RFC 822 size", and the "flags". The
internal date is the date and time that the message was placed in the
mailbox. The RFC 822 size is subject to deletion in the future; it
is the size in bytes of the message, expressed as an RFC 822 text
string. Current clients only use it as part of a status display
line. The flags are a list of status flags associated with the
message (see below). All of the first category data can be fetched
by using the macro-fetch word "FAST"; that is, "FAST" expands to
"(FLAGS INTERNALDATE RFC822.SIZE)".
The second category is that data which describes the composition and
delivery information of a message; that is, information such as the
message sender, recipient lists, message-ID, subject, etc. This is
the information which is stored in the message header in RFC 822
format message and is traditionally called the "envelope". [Note:
this should not be confused with the SMTP (RFC 821) envelope, which
is strictly limited to delivery information.] IMAP3 defines a
structured and unambiguous representation for the envelope which is
particularly nice for Lisp-based parsers. A client can use the
envelope for operations such as replying and not worry about RFC 822
at all. Envelopes are discussed in more detail below. The first and
second category data can be fetched together by using the macro-fetch
word "ALL"; that is, "ALL" expands to "(FLAGS INTERNALDATE
RFC822.SIZE ENVELOPE)".
The third category is that data which is intended for direct human
viewing. The present RFC 822 based IMAP3 defines three such items:
RFC822.HEADER, RFC822.TEXT, and RFC822 (the latter being the two
former appended together in a single text string). Fetching "RFC822"
is equivalent to typing the RFC 822 representation of the message as
stored on the mailbox without any filtering or processing.
Typically, a client will "FETCH ALL" for some or all of the messages
in the mailbox for use as a presentation menu, and when the user
wishes to read a particular message will "FETCH RFC822.TEXT" to get
the message body. A more primitive client could, of course, simply
"FETCH RFC822" a la POP2-type functionality.
The client can alter certain data by means of a STORE command. As an
example, a message is deleted from a mailbox by a STORE command which
includes the \DELETED flag as one of the flags being set.
Other client operations include copying a message to another mailbox
(COPY command), permanently removing deleted messages (EXPUNGE
command), checking for new messages (CHECK command), and searching
for messages which match certain criteria (SEARCH command).
Rice [Page 11]
RFC 1203 IMAP3 February 1991
The client terminates the session with the LOGOUT command. The
server returns a "BYE" followed by an "OK".
A Typical Scenario
Client Server
------ ------
{Wait for Connection}
{Open Connection} -->
<-- * OK IMAP3 Server Ready
{Wait for command}
A001 SUPPORTED.VERSIONS -->
<-- * SUPPORTED.VERSIONS ((2 0 )
(3 0 EIGHT.BIT.TRANSPARENT
AUTO.SET.SEEN
TAGGED.SOLICITED))
A001 OK Supported Versions returned.
{Wait for command}
A002 SELECT.VERSION (3 0) -->
<-- A002 OK Version 3.0 Selected.
{Wait for command}
A002 SELECT.FEATURES TAGGED.SOLICITED -->
<-- A002 OK Features selected.
{Wait for command}
A003 LOGIN Fred Secret -->
<-- A003 OK User Fred logged in
{Wait for command}
A004 SELECT INBOX -->
<-- A004 FLAGS (Meeting Notice \Answered
\Flagged \Deleted \Seen)
<-- A004 19 EXISTS
<-- A004 2 RECENT
<-- A004 OK Select complete
{Wait for command}
A005 FETCH 1:19 ALL -->
<-- A005 1 Fetch (......)
...
<-- A005 18 Fetch (......)
<-- A005 19 Fetch (......)
<-- A005 OK Fetch complete
{Wait for command}
A006 FETCH 8 RFC822.TEXT -->
<-- A006 8 Fetch (RFC822.TEXT {893}
...893 characters of text...
<-- )
<-- A006 OK Fetch complete
{Wait for command}
Rice [Page 12]
RFC 1203 IMAP3 February 1991
A007 STORE 8 +Flags \Deleted -->
<-- A007 8 Store (Flags (\Deleted
\Seen))
<-- A007 OK Store complete
{Wait for command}
A008 EXPUNGE -->
<-- A008 19 EXISTS
<-- A008 8 EXPUNGE
<-- A008 18 EXISTS
<-- A008 Expunge complete
{Wait for command}
A009 LOGOUT -->
<-- A009 BYE IMAP3 server quitting
<-- A009 OK Logout complete
{Close Connection} --><-- {Close connection}
{Go back to start}
A more complex scenario produced by a pipelining multiprocess client.
Client Server
------ ------
{Wait for Connection}
{Open session as above}
<-- A004 19 EXISTS
<-- A004 2 RECENT
<-- A004 OK Select complete
{Wait for command}
A005 SEARCH RECENT -->
<-- A005 SEARCH (18 19) (RECENT)
<---A005 OK Search complete
A006 FETCH 18:19 ALL RFC822.TEXT
A007 STORE 18:19 +FLAGS (\SEEN)
A008 FETCH 1:17 ALL -->
<-- A006 18 Fetch (... RFC822.TEXT ...)
A009 STORE 18 +FLAGS (\DELETED)
<-- A006 19 Fetch (... RFC822.TEXT ...)
<-- A006 OK Fetch complete
<-- A007 18 STORE (Flags (\Seen))
A010 STORE 19 +FLAGS (\DELETED)
<-- A007 19 STORE (Flags (\Seen))
<-- A007 OK Store complete
<-- A008 1 Fetch (......)
...
<-- A008 16 Fetch (......)
<-- A008 17 Fetch (......)
<-- A008 OK Fetch complete
<-- A009 18 STORE (Flags (\Seen
\Deleted))
Rice [Page 13]
RFC 1203 IMAP3 February 1991
<-- A009 OK Store complete
<-- A010 19 STORE (Flags (\Seen
\Deleted))
<-- A010 OK Store complete
{Wait for command}
<-- * EXISTS 23
<-- * RECENT 4
<-- * SEARCH (20 21 22 23) (RECENT)
A011 FETCH 20:23 ALL RFC822.TEXT
Conventions
The following terms are used in a meta-sense in the syntax
specification below:
An ASCII-STRING is a sequence of arbitrary ASCII characters.
An ATOM is a sequence of ASCII characters delimited by SP or CRLF.
A CHARACTER is any ASCII character except """", "{", CR, LF, "%",
or "\".
A CRLF is an ASCII carriage-return character followed immediately
by an ASCII linefeed character.
A NUMBER is a sequence of the ASCII characters which represent
decimal numerals ("0" through "9"), delimited by SP, CRLF, ",", or
":".
A SP is the ASCII space character.
A TEXT_LINE is a human-readable sequence of ASCII characters up to
but not including a terminating CRLF.
One of the most common fields in the IMAP3 protocol is a STRING,
which may be an ATOM, QUOTED-STRING (a sequence of CHARACTERs inside
double-quotes), or a LITERAL. A literal consists of an open brace
("{"), a number, a close brace ("}"), a CRLF, and then an ASCII-
STRING of n characters, where n is the value of the number inside the
brace. In general, a string should be represented as an ATOM or
QUOTED-STRING if at all possible. The semantics for QUOTED-STRING or
LITERAL are checked before those for ATOM; therefore an ATOM used in
a STRING may only contain CHARACTERs. Literals are most often sent
from the server to the client; in the rare case of a client to server
literal there is a special consideration (see the "+ text" response
below).
Another important field is the SEQUENCE, which identifies a set of
Rice [Page 14]
RFC 1203 IMAP3 February 1991
messages by consecutive numbers from 1 to n where n is the number of
messages in the mailbox. A sequence may consist of a single number,
a pair of numbers delimited by colon indicating all numbers between
those two numbers, or a list of single numbers and/or number pairs.
For example, the sequence 2,4:7,9,12:15 is equivalent to
2,4,5,6,7,9,12,13,14,15 and identifies all of those messages.
Definitions of Commands and Responses
Summary of Commands and Responses
Commands:
tag NOOP
tag LOGIN user password
tag LOGOUT
tag SELECT mailbox
tag CHECK
tag EXPUNGE
tag COPY sequence mailbox
tag FETCH sequence data
tag STORE sequence data value
tag SEARCH criteria
tag BBOARD bboard
tag FIND (BBOARDS / MAILBOXES) pattern
tag READONLY
tag READWRITE
tag SELECT.VERSION (major_version minor_version)
tag SELECT.FEATURES features
tag SUPPORTED.VERSIONS
tag FLAGS
tag SET.FLAGS
Responses (can be either solicited or unsolicited):
*/tag FLAGS flag_list
*/tag SEARCH (numbers) (criteria)
*/tag EXISTS
*/tag RECENT
*/tag EXPUNGE
*/tag STORE data
*/tag FETCH data
*/tag BBOARD bboard_name
*/tag MAILBOX non_inbox_mailbox_name
*/tag SUPPORTED.VERSIONS version_data
*/tag READONLY
*/tag READWRITE
*/tag OK text
*/tag NO text
*/tag BAD text
Rice [Page 15]
RFC 1203 IMAP3 February 1991
*/tag BYE text
Responses (can only be solicited):
tag COPY message_number
Responses (can only be unsolicited):
+ text
Commands
tag NOOP
The NOOP command returns an OK to the client. By itself, it does
nothing, but certain things may happen as side effects. For
example, server implementations which implicitly check the mailbox
for new mail may do so as a result of this command. The primary
use of this command is to for the client to see if the server is
still alive (and notify the server that the client is still alive,
for those servers which have inactivity autologout timers).
tag LOGIN user password
The LOGIN command identifies the user to the server and carries
the password authenticating this user. This information is used
by the server to control access to the mailboxes.
EXAMPLE: A001 LOGIN SMITH SESAME logs in as user SMITH with
password SESAME.
tag LOGOUT
The LOGOUT command indicates the client is done with the session.
The server sends a solicited BYE response before the (tagged) OK
response, and then closes the connection.
tag SELECT mailbox
The SELECT command selects a particular mailbox. The server must
check that the user is permitted read access to this mailbox.
Prior to returning an OK to the client, the server must send an
solicited FLAGS and <n> EXISTS response to the client giving the
flags list for this mailbox (simply the system flags if this
mailbox doesn't have any special flags) and the number of messages
in the mailbox. It is also recommended that the server send a <n>
RECENT unsolicited response to the client for the benefit of
clients which make use of the number of new messages in a mailbox.
It is further recommended that servers should send an unsolicited
READONLY message if the mailbox that has been selected is not
Rice [Page 16]
RFC 1203 IMAP3 February 1991
writable by the user.
Multiple SELECT commands are permitted in a session, in which case
the prior mailbox is deselected first.
The default mailbox for the SELECT command is INBOX, which is a
special name reserved to mean "the primary mailbox for this user
on this server". The format of other mailbox names is operating
system dependent (as of this writing, it reflects the path of the
mailbox on the current servers), though it could reflect any
server-specific naming convention for the namespace of mailboxes.
Such a namespace need not and should not be viewed as being
equivalent or linked to the server machine's file system.
EXAMPLES: A002 SELECT INBOX ;; selects the default mailbox.
A002 197 EXISTS ;; server says 197 messages in INBOX
A002 5 RECENT ;; server says 5 are recent.
A002 OK Select complete.
or
A003 SELECT /usr/fred/my-mail.txt
;; select a different user specified mailbox.
...
tag CHECK
The CHECK command forces a check for new messages and a rescan of
the mailbox for internal change for those implementations which
allow multiple simultaneous read/write access to the same mailbox
(e.g., TOPS-20). It is recommend that periodic implicit checks
for new mail be done by servers as well. The server must send a
solicited <n> EXISTS response prior to returning an OK to the
client.
tag EXPUNGE
The EXPUNGE command permanently removes all messages with the
\DELETED flag set in its flags from the mailbox. Prior to
returning an OK to the client, for each message which is removed,
a solicited <n> EXPUNGE response is sent indicating which message
was removed. The message number of each subsequent message in the
mailbox is immediately decremented by 1; this means that if the
last 5 messages in a 9-message mailbox are expunged you will
receive 5 "5 EXPUNGE" responses for message 5. To ensure mailbox
integrity and server/client synchronization, it is recommended
that the server do an implicit check prior to commencing the
expunge and again when the expunge is completed. Furthermore, if
the server allows multiple simultaneous access to the same mailbox
the server must guarantee both the integrity of the mailbox and
Rice [Page 17]
RFC 1203 IMAP3 February 1991
the views of it held by the clients.
EXPUNGE is not allowed if the user does not have write access to
this mailbox. If a user does not have write access to the mailbox
then the server is required to signal this fact by replying with a
NO response with a suitable text string that can be presented to
the user explaining that the mailbox is read-only. It is further
recommended that servers send an unsolicited READONLY message to
clients that attempt an expunge operation on a read only mailbox.
tag COPY sequence mailbox
The COPY command copies the specified message(s) to the specified
destination mailbox. If the destination mailbox does not exist,
the server should create it. Prior to returning an OK to the
client, the server must return a solicited <n> COPY response for
each message copied.
EXAMPLE: A003 COPY 2:4 MEETING copies messages 2, 3, and 4 to
mailbox "MEETING".
COPY is not allowed if the user does not have write access to the
destination mailbox. If a user does not have write access to the
destination mailbox then the server is required to signal this
fact by replying with a NO response with a suitable text string
that can be presented to the user explaining that the mailbox is
read-only. It is further recommended that servers send an
unsolicited READONLY message to clients that attempt to copy to a
read only mailbox. IMAP3 does not specify "where" the message
will be put in the mailbox to which it has been copied.
tag FETCH sequence fetch_att
The FETCH command retrieves data associated with a message in the
mailbox. The data items to be fetched may be either a single atom
or an S-expression list. The attributes that can be fetched are
any of those mentioned specifically below along with any generic,
canonical or concrete key. The set of predefined generic keys is:
{BCC, BODY, CC, FROM, HEADER, SIZE, SUBJECT, TEXT, TO}. The set
of predefined canonical keys is {$CC, $FROM, $SUBJECT, $TO}. The
value returned by the server for a non-existent or non-meaningful
key is defined to be the null value, NIL.
ALL Equivalent to:
(FLAGS INTERNALDATE RFC822.SIZE ENVELOPE)
ENVELOPE The envelope of the message. The envelope is
computed by the server by parsing the header,
Rice [Page 18]
RFC 1203 IMAP3 February 1991
i.e., the RFC 822 header for an RFC822 format
message, into the component parts, defaulting
various fields as necessary.
FAST Macro equivalent to:
(FLAGS INTERNALDATE RFC822.SIZE)
FLAGS The flags which are set for this message.
This may include the following system flags:
\RECENT Message arrived since
last read of this mailbox
\SEEN Message has been read
\ANSWERED Message has been answered
\FLAGGED Message is "flagged" for
urgent/special attention
\DELETED Message is "deleted" for
removal by later EXPUNGE
INTERNALDATE The date and time the message was written to
the mailbox.
RFC822 The message in RFC 822 format.
RFC822.HEADER The RFC 822 format header of the message.
RFC822.SIZE The number of characters in the message as
expressed in RFC 822 format.
RFC822.TEXT The text body of the message, omitting the
RFC 822 header.
EXAMPLES:
A003 FETCH 2:4 ALL
fetches the flags, internal date, RFC 822 size, and envelope
for messages 2, 3, and 4.
A004 FETCH 3 RFC822
fetches the RFC 822 representation for message 3.
A005 FETCH 4 (FLAGS RFC822.HEADER)
fetches the flags and RFC 822 format header for message 4.
A006 FETCH 42 $SUBJECT
A006 FETCH $SUBJECT "Some subject text..."
A006 OK FETCH completed ok.
fetches the canonical subject field.
Rice [Page 19]
RFC 1203 IMAP3 February 1991
A007 FETCH 42 APPARENTLY-TO
A007 FETCH APPARENTLY-TO NIL
A007 OK FETCH found no value.
fetches the concrete apparently-to field.
tag STORE sequence data value
The STORE command alters the values associated with particular
keys for a message in the mailbox. As is the case for the FETCH
command, any generic, canonical or concrete key may be used to
index the value provided. In addition to these, the following
pre-defined keys are provided.
FLAGS Replace the flags for the message with the
argument (in flag list format).
The server must respond with a solicited STORE FLAGS
message, showing the new state of the flags after
the store.
+FLAGS Add the flags in the argument to the
message's flag list.
The server must respond with a solicited STORE FLAGS
message, showing the new state of the flags after
the store.
-FLAGS Remove the flags in the argument from the
message's flag list.
The server must respond with a solicited STORE FLAGS
message, showing the new state of the flags after
the store.
RFC822.HEADER Replace the header of the message(s) with that
specified. This allows users to use their mailboxes
as databases with header fields as keys.
The server must respond with solicited
STORE RFC822.HEADER, STORE RFC822.SIZE and
STORE ENVELOPE messages, showing the new state
of the reparsed header after the store.
RFC822.TEXT Replace the body of the messages with that specified.
The server must respond with solicited
STORE RFC822.TEXT and STORE RFC822.SIZE messages,
showing the new state of the message after the store.
STORE is not allowed if the user does not have write access to
this mailbox.
The server is required to send a solicited STORE response for
Rice [Page 20]
RFC 1203 IMAP3 February 1991
each store operation that results in a format transformation by
the server. For example, the server is required to send a
STORE FLAGS response when the client performs a STORE +FLAGS or
a STORE -FLAGS, since the client may not easily be able to know
what the result of this command will be. Similarly, if the
client emits a STORE FROM command then the server should
respond with a suitable STORE FROM response because the client
would be sending a string value to be stored and the server
should transform this into a set of addresses. In general,
however, although it is legal for the server to send a
solicited STORE response for each STORE operation, this is
discouraged, since it might result in the retransmission of
very large and unnecessary amounts of data that have been
stored.
EXAMPLE: A003 STORE 2:4 +FLAGS (\DELETED) marks messages 2, 3,
and 4 for deletion.
tag SEARCH search_criteria
The SEARCH command searches the mailbox for messages which match
the given set of criteria. The server response SEARCH (criteria)
(numbers) gives the set of messages which match the conjunction of
the criteria specified. In addition to each of the search
criteria there is its logical inverse. The logical inverse
criterion is denoted by the ~ (tilda) sign.
Thus, no message that matches the criterion:
FROM crispin
will match the criterion:
~FROM crispin
The criteria for the search can be any generic, canonical or
concrete key. In addition to these, the following pre-defined
keys are also provided:
ALL All messages in the mailbox; the default
initial criterion for ANDing.
ANSWERED Messages with the \ANSWERED flag set.
BCC string Messages which contain the specified string
in the envelope's BCC field.
BEFORE date Messages whose internal date is earlier than
the specified date.
Rice [Page 21]
RFC 1203 IMAP3 February 1991
BODY string Messages which contain the specified string
in the body of the message.
CC string Messages which contain the specified string
in the envelope's CC field.
DELETED Messages with the \DELETED flag set.
FLAGGED Messages with the \FLAGGED flag set.
FROM string Messages which contain the specified string
in the envelope's FROM field.
HEADER string Messages which contain the specified string
in the message header.
KEYWORD flag Messages with the specified flag set.
NEW Messages which have the \RECENT flag set but
not the \SEEN flag. This is functionally
equivalent to "RECENT UNSEEN".
OLD Messages which do not have the \RECENT flag
set.
ON date Messages whose internal date is the same as
the specified date.
RECENT Messages which have the \RECENT flag set.
SEEN Messages which have the \SEEN flag set.
SINCE date Messages whose internal date is later than
the specified date.
SUBJECT string Messages which contain the specified string
in the envelope's SUBJECT field.
TEXT string Messages which contain the specified string.
TO string Messages which contain the specified string in
the envelope's TO field.
EXAMPLE: A003 SEARCH DELETED FROM "SMITH" SINCE 1-OCT-87
returns the message numbers for all deleted messages from Smith
that were placed in the mailbox since October 1, 1987.
Implementation note: The UNANSWERED, UNDELETED, UNFLAGGED,
Rice [Page 22]
RFC 1203 IMAP3 February 1991
UNKEYWORD and UNSEEN criteria, described below, are preserved in
IMAP3 for IMAP2 compatibility. They are, however, considered
obsolete and new Client programs are encouraged to use the ~
notation for the logical inverses of search criteria with a view
to the dropping of this outmoded syntax in later versions.
UNANSWERED Messages which do not have the \ANSWERED flag
set.
UNDELETED Messages which do not have the \DELETED flag
set.
UNFLAGGED Messages which do not have the \FLAGGED flag
set.
UNKEYWORD flag Messages which do not have the specified flag
set.
UNSEEN Messages which do not have the \SEEN flag set.
tag READONLY
The READONLY command indicates that the client wishes to make the
mailbox read-only. The server is required to reply with a
solicited READONLY or READWRITE response.
tag READWRITE
The READWRITE command indicates that the client wishes to make the
mailbox read-write. The server is required to reply with a
solicited READONLY or READWRITE response.
tag SUPPORTED.VERSIONS
The SUPPORTED.VERSIONS solicits from the server a
SUPPORTED.VERSIONS message, which encapsulates information about
which versions and features the server supports.
tag SELECT.VERSION (major_version minor_version)
The SELECT.VERSION command indicates that the client wishes to
select certain behavior on the part of the server. The major and
minor versions indicate the specific version of the protocol being
selected.
EXAMPLE: A002 SELECT.VERSION (3 0)
A client may not request a server version that is not supported by
Rice [Page 23]
RFC 1203 IMAP3 February 1991
the server, i.e., which is specifically mentioned in the response
to a SUPPORTED.VERSIONS command. An attempt to do so by a client
will result in a NO response from the server. It is an error for
the SELECT.VERSION command to be used after a mailbox has been
selected. The rationale for this is that for some server
implementations it might be necessary to spawn separate programs
to implement widely divergent protocol versions. Thus, the client
cannot be allowed to expect any server state to be preserved after
the use of the SELECT.VERSION command. The default version of all
servers is 2.0, i.e., IMAP2 as defined by RFC 1064.
tag SELECT.FEATURES 1#features
The SELECT.FEATURES command indicates that the client wishes to
select certain specific features on the part of the server. A
client may not request a feature that is not supported by the
server, i.e., one that is explicitly mentioned in the set of
features for the selected version returned by the
SUPPORTED.VERSIONS command. An attempt to do so by a client will
result in a NO response from the server.
EXAMPLE: A002 SELECT.FEATURES AUTO.SET.SEEN ~TAGGED.SOLICITED
EIGHT.BIT.TRANSPARENT
i.e., select the set of features called AUTO.SET.SEEN and
EIGHT.BIT.TRANSPARENT and deselect the feature called
TAGGED.SOLICITED. The use of the SELECT.FEATURES command
completely resets the set of selected features. Note: These are
only example feature names and are not necessarily supported by
any server. See the appendix on features for more information on
features. Note: Some features, when present in the server, will
cause the upwards compatible extension of the grammar, i.e., by
adding extra commands. The server is at liberty not to remove
these upwards compatible extensions to the command tables when a
feature is disabled. Thus, it is an error for a client to rely on
getting a NO or BAD response in any way, for instance to determine
the selectedness or presence of a feature.
tag BBOARD bboard
The BBOARD command is equivalent to SELECT, except that its
argument is a bulletin board (BBoard) name. The format of a
BBoard name is implementation specific, although it is strongly
encouraged to use something that resembles a name in a generic
sense and not a file or mailbox name on the particular system.
There is no requirement that a BBoard name be a mailbox name or a
file name (in particular, Unix netnews has a completely different
namespace from mailbox or file names).
Rice [Page 24]
RFC 1203 IMAP3 February 1991
The result from the BBOARD command is identical from that of the
SELECT command. For example, in the TOPS-20 server
implementation, the command
A0002 BBOARD FOO
is exactly equivalent to the command
A0002 SELECT POBOX:<BBOARD>FOO.TXT
Note: the equivalence in this example is *not* required by the
protocol, and merely reflects the fuzzy distinction between
mailboxes and BBoards on TOPS-20.
tag FIND (BBOARDS / MAILBOXES) pattern
The FIND command accepts as arguments the keywords BBOARDS or
MAILBOXES and a pattern which specifies some set of BBoard/mailbox
names which are usable by the BBOARD/SELECT command. Two wildcard
characters are defined; "*" specifies that any number (including
zero) characters may match at this position and "%" specifies that
a single character may match at this position. For example,
FOO*BAR will match FOOBAR, FOOD.ON.THE.BAR and FOO.BAR, whereas
FOO%BAR will match only FOO.BAR; furthermore, "*" will match all
BBoards/mailboxes. The following quoting convention applies to
wildcards: "\*" is the literal "*" character, "\%" is the literal
"%" character and "\\" is the literal "\" character. Notes: The
format of mailboxes is server implementation dependent. The
special mailbox name INBOX is not included in the output to the
FIND MAILBOXES command.
The FIND command solicits any number of BBOARD or MAILBOX
responses from the server as appropriate.
Examples:
A0002 FIND BBOARDS *
A0002 BBOARD FOOBAR
A0002 BBOARD GENERAL
A0002 OK FIND completed
or
A0002 FIND MAILBOXES FOO%BA*
A0002 MAILBOX FOO.BAR
A0002 MAILBOX FOO.BAZZAR
A0002 OK FIND completed
Note: Although the use of explicit file or path names for
mailboxes is discouraged by this standard, it may be unavoidable.
It is important that the value returned in the MAILBOX solicited
reply be usable in the SELECT command without remembering any path
specification which may have been used in the FIND MAILBOXES
pattern.
Rice [Page 25]
RFC 1203 IMAP3 February 1991
tag FLAGS
The FLAGS command solicits a FLAGS response from the server.
tag SET.FLAGS flag_list
The SET.FLAGS command defines the user specifiable flags for this
mailbox, i.e., the keywords. If this set does not include flags
formerly sent to the client by the server in a FLAGS message then
this constitutes a request to delete the flag. Any new flags
should be created. This command does not affect the system
defined flags and any system flags that are included in the
flag_list will be ignored. The server must respond to this
command with a solicited FLAGS message. If the deletion of a flag
results in the invalidation of the flag sets of any messages then
the server is required to send solicited STORE FLAGS messages to
the client for each modified message.
Responses:
*/tag OK text
In its solicited form this response identifies successful
completion of the command with the indicated tag. The text is a
line of human-readable text which may be useful in a protocol
telemetry log for debugging purposes.
In its unsolicited form, this response indicates simply that the
server is alive. No special action on the part of the client is
called for. This is presently only used by servers at startup as
a greeting message indicating that they are ready to accept the
first command. This usage, although legal, is by no means
required. The text is a line of human-readable text which may be
logged in protocol telemetry.
*/tag NO text
In its solicited form this response identifies unsuccessful
completion of the command with the indicated tag. The text is a
line of human-readable text which probably should be displayed to
the user in an error report by the client.
In its unsolicited form this response indicates some operational
error at the server which cannot be traced to any protocol
command. The text is a line of human-readable text which should
be logged in protocol telemetry for the maintainer of the server
and/or the client.
Rice [Page 26]
RFC 1203 IMAP3 February 1991
*/tag BAD text
In its solicited form response indicates faulty protocol received
from the client and indicates a bug. The text is a line of
human-readable text which should be recorded in any telemetry as
part of a bug report to the maintainer of the client.
In its unsolicited form response indicates some protocol error at
the server which cannot be traced to any protocol command. The
text is a line of human-readable text which should be logged in
protocol telemetry for the maintainer of the server and/or the
client. This generally indicates a protocol synchronization
problem, and examination of the protocol telemetry is advised to
determine the cause of the problem.
*/tag BYE text
This indicates that the server is about to close the connection.
The text is a line of human-readable text which should be
displayed to the user in a status report by the client. IMAP2
requires that the server emit a solicited BYE response as part of
a normal logout sequence. This solicited form is not required
under IMAP3, though is still legal for compatibility. In its
unsolicited form the BYE response is used as a panic shutdown
announcement by the server. It is required to be used by any
server which performs autologouts due to inactivity.
*/tag number message_data
The solicited (tag number message_data) response is generated as
the result of a number of client requests. The server may also
emit any the following at any time as unsolicited data (i.e., *
number message_data). The message_data is one of the following:
EXISTS The specified number of messages exists in the mailbox.
RECENT The specified number of messages have arrived since the
last time this mailbox was selected with the SELECT
command or equivalent.
EXPUNGE The specified message number has been permanently
removed from the mailbox, and the next message in the
mailbox (if any) becomes that message number.
The server must send a solicited EXPUNGE response
for each message that it expunges as the result
of an EXPUNGE command. Note: future versions of the
protocol may allow the use of a message sequence
as a value returned by the EXPUNGE response to allow the
Rice [Page 27]
RFC 1203 IMAP3 February 1991
more efficient compaction of client representations of
mailboxes.
STORE data
Functionally equivalent to FETCH, only it is sent by the
server when the state of a mailbox changes. The server
must send solicited STORE responses as the result of
any change caused by a STORE command.
FETCH data
This is the principle means by which data about a
message is sent to the client. The data is in a
Lisp-like S-expression property list form. Just as the
FETCH request from the client can fetch any generic,
canonical or concrete key, so also the FETCH response
can return values for any of these keys as well as for
the pre-defined attributes mentioned below. Note that
the server is permitted to send any unsolicited FETCH
or STORE messages that it should choose, be they the
values associated with generic, canonical or concrete
keys. Clients are required to ignore any such
FETCH responses that it cannot interpret. For example,
clients are not required to be able to understand, i.e.,
use fruitfully, the canonical $TO key, but they are
required to be able to ignore an unsolicited $TO message
correctly.
ENVELOPE An S-expression format list which describes the
envelope of a message. The envelope is computed
by the server by parsing the RFC 822 header into
the component parts, defaulting various fields
as necessary.
The fields of the envelope are in the following
order: date, subject, from, sender, reply-to, to,
cc, bcc, in-reply-to, and message-id. The date,
subject, in-reply-to, and message-id fields are
strings. The from, sender, reply-to, to, cc,
and bcc fields are lists of addresses.
An address is an S-expression format list which
describes an electronic mail address. The fields
of an address are in the following order:
personal name, source-route (i.e., the
at-domain-list in SMTP), mailbox name, host name
and comments. Implementation note: The addition
of the comment field is an incompatible extension
from IMAP2. The server is required not to provide
Rice [Page 28]
RFC 1203 IMAP3 February 1991
this field when running in IMAP2 mode.
Any field of an envelope or address which is
not applicable is presented as the atom NIL.
Note that the server must default the reply-to
and sender fields from the from field; a client is
not expected to know to do this.
FLAGS An S-expression format list of flags which are set
for this message. This may include the following
system flags:
\RECENT Message arrived since last
read of this mailbox
\SEEN Message has been read
\ANSWERED Message has been answered
\FLAGGED Message is "flagged" for
urgent/special attention
\DELETED Message is "deleted" for
removal by later EXPUNGE
INTERNALDATE A string containing the date and time the
message was written to the mailbox.
RFC822 A string expressing the message in RFC 822
format.
Note: Some implementations of IMAP2 servers
had the (undocumented) behavior of setting
the \SEEN flag as a side effect of fetching
the body of a message. This resulted in
erroneous behavior for clients that prefetch
messages that the user might not get
around to reading. Thus, this behavior is
explicitly disallowed in IMAP3.
Note: this is not a significant performance
restriction because it is always possible for
IMAP3 clients to use an interaction with the
server of the following type:
A001 FETCH 42 RFC822
A002 STORE 42 +FLAGS (\SEEN)
A001 42 FETCH RFC822 {637} ......
A001 OK Fetch completed
A002 42 STORE FLAGS (\SEEN \FLAGGED...)
A002 OK Store Completed.
RFC822.HEADER A string expressing the RFC 822 format
header of the message
Rice [Page 29]
RFC 1203 IMAP3 February 1991
RFC822.SIZE A number indicating the number of
characters in the message as expressed
in RFC 822 format.
RFC822.TEXT A string expressing the text body of the
message, omitting the RFC 822 header.
See also note for RFC822.
*/tag FLAGS flag_list
A solicited FLAGS response must occur as a result of a SELECT
command. The flag list is the list of flags (at a minimum, the
IMAP defined flags) which are applicable for this mailbox. Flags
other than the system flags are a function of the server
implementation.
*/tag SEARCH (numbers) (search_criteria)
This response occurs as a result of a SEARCH command. The
number(s) refer to those messages which match the search criteria.
In its solicited form this message allows clients to find
interesting groups of messages, e.g., unseen messages from
Crispin. In its unsolicited form it allows the server to inform
the client of interesting patterns, e.g., when new mail arrives,
recent and from Crispin. Compatibility note: The search_criteria
are sent by the server along with the matching numbers so
unsolicited SEARCH messages may be interpreted. This syntax is
not upwards compatible with IMAP2 and so the new syntax is
intended to make it simple for clients that are not able to take
advantage of unsolicited SEARCH messages still to interpret
solicited SEARCH messages simply by ignoring everything that
follows the list of numbers with minimal parsing. Such clients
may not, however, simply discard the rest of the line because
there might be LITERALs in the search pattern.
Examples:
A00042 SEARCH (2 3 6) (FROM Crispin ~SEEN)
and
* SEARCH (42) (FROM Crispin RECENT)
*/tag READONLY
This indicates that the mailbox is read-only. The server is
required to respond to a READONLY or READWRITE command with either
a solicited READONLY or a solicited READWRITE response. Note: If
the client attempts a mutation operation, such as STORE, on a
mailbox to which it does not have write access then the server is
required to reply with a solicited READONLY response on the first
Rice [Page 30]
RFC 1203 IMAP3 February 1991
such attempted mutation. The server may also choose to send
solicited READONLY responses for each subsequent attempted
mutation.
*/tag READWRITE
This indicates that the mailbox is read-write. The server is
required to respond to a READONLY or READWRITE command with either
a solicited READONLY or a solicited READWRITE response.
*/tag BBOARD bboard_name
This message is produced in its solicited form as a response to a
FIND BBOARDS command. In its unsolicited form it represents a
notification by the server that a new BBoard has been added.
Bboard_name must be a name that can be supplied to the BBOARD
command so as to select the appropriate bboard.
*/tag MAILBOX non_inbox_mailbox_name
This message is produced in its solicited form as a response to a
FIND MAILBOXES command. In its unsolicited form it represents a
notification by the server that a new mailbox has been added,
perhaps as the result of a COPY command creating a new mailbox.
Non_inbox_mailbox_name must be a name that can be supplied to the
SELECT command so as to select the appropriate mailbox. Note:
non_inbox_mailbox_name is never the string "INBOX".
*/tag SUPPORTED.VERSIONS (version_specs)
This message is used either as a response to the
SUPPORTED.VERSIONS or, in its unsolicited form, to indicate the
dynamic addition or removal of support for features or protocol
versions. Each version_spec is of the form (4 2
EIGHT.BIT.TRANSPARENT AUTO.SET.SEEN ...), i.e., a major version
number and a minor version number for the protocol and the set of
features supported under the server's implementation of that
protocol version. A server may not dynamically remove support for
any version or feature that has been selected by any currently
logged in client by the use of the VERSION command.
Example:
A00005 SUPPORTED.VERSIONS ((2 0 )
(2 2 TAGGED.SOLICITED)
(3 0 EIGHT.BIT.TRANSPARENT TAGGED.SOLICITED))
Indicates that two major versions are supported and one minor
version is supported and that tagged solicited messages are
Rice [Page 31]
RFC 1203 IMAP3 February 1991
supported in versions 2.2 and 3.0 with eight bit characters being
supported under version 3. For each feature mentioned in the list
of features there is also always the negation of that feature.
For example, if the server supports the TAGGED.SOLICITED feature
then it also supports the ~TAGGED.SOLICITED feature, which
disables this feature. Note: These are only example feature
names and are not necessarily supported by any server. See the
appendix on features for more information on features.
+ text
This response indicates that the server is ready to accept the
text of a literal from the client. Normally, a command from the
client is a single text line. If the server detects an error in
the command, it can simply discard the remainder of the line. It
cannot do this in the case of commands which contain literals,
since a literal can be an arbitrarily long amount of text, and the
server may not even be expecting a literal. This mechanism is
provided so the client knows not to send a literal until the
server definitely expects it, preserving client/server
synchronization.
In actual practice, this situation is rarely encountered. In the
current protocol, the only client commands likely to contain
literals are the LOGIN command and the STORE RFC822.HEADER or
STORE RFC822.TEXT commands. Consider a situation in which a
server validates the user before checking the password. If the
password contains "funny" characters and hence is sent as a
literal, then if the user is invalid an error would occur before
the password is parsed.
No such synchronization protection is provided for literals sent
from the server to the client, for performance reasons. Any
synchronization problems in this direction would be due to a bug
in the client or server and not for some operational problem.
Sample IMAP3 session
The following is a transcript of an actual IMAP3 session. Server
output is identified by "S:" and client output by "U:". In cases
where lines were too long to fit within the boundaries of this
document, the line was continued on the next line preceded by a tab.
S: * OK SUMEX-AIM.Stanford.EDU Interactive Mail Access Protocol
III Service 6.1(349) at Mon, 14 May 90 14:58:30 PDT
U: a001 SUPPORTED.VERSIONS
S: * SUPPORTED.VERSIONS ((2 0 ) (3 0 EIGHT.BIT.TRANSPARENT
AUTO.SET.SEEN TAGGED.SOLICITED))
Rice [Page 32]
RFC 1203 IMAP3 February 1991
S: A001 Supported Versions returned.
U: a002 SELECT.VERSION (3 0)
S: a002 OK Version 3.0 Selected.
U: a003 SELECT.FEATURES TAGGED.SOLICITED
S: a003 OK Features selected.
U: a004 login crispin secret
S: a004 OK User CRISPIN logged in at Thu, 9 Jun 90 14:58:42 PDT,
job 76
U: a005 select inbox
S: a005 FLAGS (Bugs SF Party Skating Meeting Flames Request AI
Question Note \XXXX \YYYY \Answered \Flagged \Deleted
\Seen)
S: a005 16 EXISTS
S: a005 0 RECENT
S: a006 OK Select complete
U: a006 fetch 16 all
S: a006 16 Fetch (Flags (\Seen) InternalDate " 9-Jun-88 12:55:
RFC822.Size 637 Envelope ("Sat, 4 Jun 88 13:27:11 PDT"
"INFO-MAC Mail Message" (("Larry Fagan" NIL "FAGAN"
"SUMEX-AIM.Stanford.EDU" NIL)) (("Larry Fagan" NIL "FAGAN"
"SUMEX-AIM.Stanford.EDU" NIL)) (("Larry Fagan" NIL "FAGAN"
"SUMEX-AIM.Stanford.EDU" NIL)) ((NIL NIL "rindflEISCH"
"SUMEX-AIM.Stanford.EDU" NIL)) NIL NIL NIL
"<12403828905.13.FAGAN@SUMEX-AIM.Stanford.EDU>"))
S: a006 OK Fetch completed
U: a007 fetch 16 rfc822
S: a007 16 Fetch (RFC822 {637}
S: Mail-From: RINDFLEISCH created at 9-Jun-88 12:55:43
S: Mail-From: FAGAN created at 4-Jun-88 13:27:12
S: Date: Sat, 4 Jun 88 13:27:11 PDT
S: From: Larry Fagan <FAGAN@SUMEX-AIM.Stanford.EDU>
S: To: rindflEISCH@SUMEX-AIM.Stanford.EDU
S: Subject: INFO-MAC Mail Message
S: Message-ID: <12403828905.13.FAGAN@SUMEX-AIM.Stanford.EDU>
S: ReSent-Date: Thu, 9 Jun 88 12:55:43 PDT
S: ReSent-From: TC Rindfleisch <Rindfleisch@SUMEX-AIM.Stanford.EDU>
S: ReSent-To: Yeager@SUMEX-AIM.Stanford.EDU,
Crispin@SUMEX-AIM.Stanford.EDU
S: ReSent-Message-ID:
<12405133897.80.RINDFLEISCH@SUMEX-AIM.Stanford.EDU>
S:
S: The file is <info-mac>usenetv4-55.arc ...
S: Larry
S: -------
S: )
S: a007 OK Fetch completed
U: a008 logout
S: a008 BYE UNIX IMAP III server terminating connection
Rice [Page 33]
RFC 1203 IMAP3 February 1991
S: a008 OK SUMEX-AIM.Stanford.EDU Interim Mail Access Protocol
Service logout
Implementation Discussion
As of this writing, SUMEX has completed an IMAP2 client for Xerox
Lisp machines written in hybrid Interlisp/CommonLisp and is beginning
distribution of a client for TI Explorer Lisp machines. SUMEX has
also completed a portable IMAP2 client protocol library module
written in C. This library, with the addition of a small main
program (primarily user interface) and a TCP/IP driver, became a
rudimentary remote system mail-reading program under Unix. The first
production use of this library is as a part of a MacII client which
has now been under daily use (by real users) at Stanford for quite
some time.
As of this writing, SUMEX has completed IMAP2 servers for TOPS-20
written in DEC-20 assembly language and 4.2/3 BSD Unix written in C.
The TOPS-20 server is fully compatible with MM-20, the standard
TOPS-20 mailsystem, and requires no special action or setup on the
part of the user. The INBOX under TOPS-20 is the user's MAIL.TXT.
The TOPS-20 server also supports multiple simultaneous access to the
same mailbox, including simultaneous access between the IMAP3 server
and MM-20. The 4.2/3 BSD Unix server requires that the user use
either Unix Mail format or mail.txt format which is compatible with
SRI MM-32 or Columbia MM-C. The 4.2/3 BSD Unix server allows
simultaneous read access; write access must be exclusive. There is
also an experimental IMAP3 server running on the TI Explorer class of
machine, which uses MM mailbox format and which can communicate over
both TCP and Chaos.
The Xerox Lisp client and DEC-20 server have been in production use
for over two years; the Unix server was been in production use for
over a year. IMAP3 has been used to access mailboxes at remote sites
from a local workstation via the Internet. For example, from the
Stanford local network one of the authors has read his mailbox at a
Milnet site.
A number of IMAP clients have now been developed or are being
developed. Amongst these are versions that run on the following
machines:
. Xerox Lisp machines
. Apple Macintosh
. NeXT
. IBM PC
. TI Explorer Lisp machines
. "Glass teletype" version that runs under Unix
Rice [Page 34]
RFC 1203 IMAP3 February 1991
. GNU Emacs
. X Windows
. NTT ELIS
Each of these client programs is carefully tuned to optimize the
performance and user interface in a manner that is consistent with
the the user interface model of the native machine. For example, the
Macintosh client features a "messy-desk" interface that allows the
cutting and pasting of text with the use of the clipboard with a menu
driven interface with keyboard accelerators.
This specification does not make any formal definition of size
restrictions, but some of the existing servers have the following
limitations:
DEC-20
. length of a mailbox: 7,077,888 characters
. maximum number of messages: 18,432 messages
. length of a command line: 10,000 characters
. length of the local host name: 64 characters
. length of a "short" argument: 39 characters
. length of a "long" argument: 491,520 characters
. maximum amount of data output in a single fetch:
655,360 characters
TI-Explorer
. length of a mailbox: limited by the Minimum of the size of the
virtual address space and the size of the file system
. maximum number of messages: limited by the the size of the
virtual address space
. length of a command line: limited by the the size of the
virtual address space
. length of the local host name: limited by the the size of the
virtual address space
. length of a "short" argument: limited by the the size of the
virtual address space
. length of a "long" argument: limited by the the size of the
virtual address space
. maximum amount of data output in a single fetch: not limited
Typical values for these limits are 30Mb for file systems and 128Mb
for virtual address space.
To date, nobody has run up against any of these limitations, many of
which are substantially larger than most current user mail reading
programs.
There are several advantages to the scheme of tags and solicited
Rice [Page 35]
RFC 1203 IMAP3 February 1991
responses and unsolicited data. First, the infamous synchronization
problems of SMTP and similar protocols do not happen with tagged
commands; a command is not considered satisfied until a completion
acknowledgement with the same tag is seen. Tagging allows an
arbitrary amount of other responses ("solicited" data) to be sent by
the server with no possibility of the client losing synchronization.
Compare this with the problems that FTP or SMTP clients have with
continuation, partial completion, and commentary reply codes.
Another advantage is that a non-lockstep client implementation is
possible. The client could send a command, and entrust the handling
of the server responses to a different process which would signal the
client when the tagged response comes in. Some clients might be
implemented in a thoroughly asynchronous manner, having, perhaps,
multiple outstanding commands at any given time. Note: this does
not require that the server process these commands in anything other
than a lock-step manner. It simply allows clients to take advantage
of servers that are able to do such asynchronous operations.
It was observed that synchronization problems can occur with literals
if the literal is not recognized as such. Fortunately, the cases in
which this can happen are relatively rare; a mechanism (the special
"+" tag response) was introduced to handle those few cases which
could happen. The proper way to address this problem in all cases is
probably to move towards a record-oriented architecture instead of
the text stream model provided by TCP.
Unsolicited data needs some discussion. Unlike most protocols, in
which the server merely does the client's bidding, an IMAP3 server
has a semi-autonomous role. By means of sending "unsolicited data",
the server is in effect sending a command to the client -- to update
and/or extend its (incomplete) model of the mailbox with new
information from the server. In this viewpoint, although a "fetch"
command is a request for specific information from the client, the
server is always at liberty to include more than the desired data as
"unsolicited". A server acknowledgement to the "fetch" is a
statement that at least all the requested data has been sent.
In terms of implementation, a simple lock-step client may have a
local cache of data from the mailbox. This cache is incomplete in
general, and at select time is empty. A listener on the IMAP
connection in the client processes all solicited and unsolicited data
symmetrically, and updates the cache based on this data, i.e., the
client faults on a cache miss and asks the server to fill that cache
slot synchronously. If a tagged completion response arrives, the
listener unblocks the process which sent the tagged request.
Clearly, given this model it is not strictly necessary to distinguish
Rice [Page 36]
RFC 1203 IMAP3 February 1991
most solicited from unsolicited data. Doing so, however, apart from
being clearer, also allows such simplistic, lock-step client
implementations that extract the specific value of the response to
command by trapping the tagged response. This allows the client not
to have to block on some complex predicate that involves watching to
see an update in a cache cell.
For example, perhaps as a result of opening a mailbox, solicited data
from the server arrives. The first piece of data is the number of
messages. This is used to size the cache; note that, if new mail
arrives, by sending a new "number of messages" unsolicited data
message server will cause the cache to be re-sized. If the client
attempts to access information from the cache, it will encounter
empty spots which will trigger "fetch" requests. The request would
be sent, some solicited data including the answer to the fetch will
flow back, and then the "fetch" response will unblock the client.
People familiar with demand-paged virtual memory design will
recognize this model as being very similar to page-fault handling on
a demand-paged system.
Formal Syntax
The following syntax specification uses the augmented Backus-Naur
Form (BNF) notation as specified in RFC 822 with one exception; the
delimiter used with the "#" construct is a single space (SP) and not
a comma.
address ::= "(" addr_name SP addr_adl SP addr_mailbox SP
addr_host addr_comment ")"
addr_adl ::= nil / string
addr_comment ::= nil / string
addr_host ::= nil / string
addr_mailbox ::= nil / string
addr_name ::= nil / string
bboard ::= "BBOARD" SP bboard_name
bboard_name ::= string
bboard_notify ::= "BBOARD" sp bboard_name
canonical_key ::= "$CC" / "$FROM" / "$SUBJECT" / "$TO"
Rice [Page 37]
RFC 1203 IMAP3 February 1991
check ::= "CHECK"
concrete_key ::= string
copy ::= "COPY" SP sequence SP mailbox
criterion ::= "ALL" / "ANSWERED" /
"BCC" SP string / "BEFORE" SP string /
"BODY" SP string / "CC" SP string / "DELETED" /
"FLAGGED" / "KEYWORD" SP atom / "NEW" / "OLD" /
"ON" SP string / "RECENT" / "SEEN" /
"SINCE" SP string / "TEXT" SP string /
"TO" SP string / "UNANSWERED" / "UNDELETED" /
"UNFLAGGED" / "UNKEYWORD" / "UNSEEN" / key SP string
criteria ::= 1#criterion
data ::= ("FLAGS" SP flag_list /
search_notify / bboard_notify / mailbox_notify /
supported_versions_notify / "READONLY" / "READWRITE" /
"BYE" SP text_line / "OK" SP text_line /
"NO" SP text_line / "BAD" SP text_line)
date ::= string in form "dd-mmm-yy hh:mm:ss-zzz"
envelope ::= "(" env_date SP env_subject SP env_from SP
env_sender SP env_reply-to SP env_to SP
env_cc SP env_bcc SP env_in-reply-to SP
env_message-id ")"
env_bcc ::= nil / "(" 1*address ")"
env_cc ::= nil / "(" 1*address ")"
env_date ::= string
env_from ::= nil / "(" 1*address ")"
env_in-reply-to ::= nil / string
env_length ::= NUMBER
env_message-id ::= nil / string
env_reply-to ::= nil / "(" 1*address ")"
env_sender ::= nil / "(" 1*address ")"
Rice [Page 38]
RFC 1203 IMAP3 February 1991
env_subject ::= nil / string
env_to ::= nil / "(" 1*address ")"
expunge ::= "EXPUNGE"
feature ::= ATOM
fetch ::= "FETCH" SP sequence SP ("ALL" / "FAST" /
fetch_att / "(" 1#fetch_att ")")
fetch_att ::= "ENVELOPE" / "FLAGS" / "INTERNALDATE" /
"RFC822" / "RFC822.HEADER" / "RFC822.SIZE" /
"RFC822.TEXT" / key
find ::= "FIND" ("BBOARDS" / "MAILBOXES") pattern
flag_list ::= ATOM / "(" 1#ATOM ")"
flags ::= "FLAGS"
generic_key ::= "BCC" / "BODY" / "CC" / "FROM" / "HEADER" / "SIZE" /
"SUBJECT" / "TEXT" / "TO"
key ::= generic_key / canonical_key / concrete_key
literal ::= "{" NUMBER "}" CRLF ASCII-STRING
login ::= "LOGIN" SP userid SP password
logout ::= "LOGOUT"
mailbox ::= "INBOX" / string
mailbox_notify ::= MAILBOX non_inbox_mailbox_name
msg_copy ::= "COPY"
msg_data ::= (msg_exists / msg_recent / msg_expunge /
msg_fetch / msg_copy)
msg_exists ::= "EXISTS"
msg_expunge ::= "EXPUNGE"
msg_fetch ::= ("FETCH" / "STORE") SP "(" 1#("ENVELOPE" SP
env_length envelope / "FLAGS" SP "(" 1#(recent_flag
flag_list) ")" / "INTERNALDATE" SP date /
Rice [Page 39]
RFC 1203 IMAP3 February 1991
"RFC822" SP string / "RFC822.HEADER" SP string /
"RFC822.SIZE" SP NUMBER / "RFC822.TEXT" SP
string / key SP string_list) ")"
msg_recent ::= "RECENT"
msg_num ::= NUMBER
nil ::= "NIL"
non_inbox_mailbox_name ::= string
noop ::= "NOOP"
numbers ::= 1#NUMBER
password ::= string
pattern ::= string
recent_flag ::= "\RECENT"
read_only ::= "READONLY"
read_write ::= "READWRITE"
ready ::= "+" SP text_line
request ::= tag SP (noop / login / logout / select / check /
expunge / copy / fetch / store / search /
select_version / select_features /
supported_versions / bboard / find /
read_only / read_write / flags / set_flags ) CRLF
response ::= tag SP ("OK" / "NO" / "BAD") SP text_line CRLF
search ::= "SEARCH" SP criteria
search_notify ::= "SEARCH" SP (numbers) SP (criteria)
select ::= "SELECT" SP mailbox
select_features ::= "SELECT.FEATURES" 1#feature
select_version ::= "SELECT.VERSION" SP "(" NUMBER SP NUMBER ")"
sequence ::= NUMBER / (NUMBER "," sequence) / (NUMBER ":"
sequence)
Rice [Page 40]
RFC 1203 IMAP3 February 1991
set_flags ::= "SET.FLAGS" SP flag_list
solicited ::= tag SP (msg_num SP msg_data / data /
solicited_only) CRLF
solicited_only ::= {None currently defined}
store ::= "STORE" SP sequence SP store_att
store_att ::= ("+FLAGS" SP flag_list / "-FLAGS" SP flag_list /
"FLAGS" SP flag_list / RFC822.TEXT SP string
/ RFC822.HEADER SP string / key SP string)
string ::= atom / """" 1*character """" / literal
string_list ::= string / ("(" 1#string ")")
supported_versions ::= "SUPPORTED.VERSIONS"
supported_versions_notify ::= "SUPPORTED.VERSIONS" "(" 1#version_spec
")"
system_flags ::= "\ANSWERED" SP "\FLAGGED" SP "\DELETED" SP
"\SEEN"
tag ::= atom
unsolicited ::= "*" SP (msg_num SP msg_data / data) CRLF
userid ::= string
version_spec ::= "(" NUMBER SP NUMBER SP 1#feature ")"
Appendix: Features.
In this section we outline the standard features that are supported
by all IMAP3 servers and identify those features which are
recommended or experimental. For each of these features the default
setting is specified. This means that it is required of any server
that supports a given feature to make the default enabledness of that
feature as is specified below. It is required that for each feature
supported by a server the inverse feature should also be supported.
The inverse feature name shall always be defined as the feature name
preceded by the "~" character. Thus, the AUTO.SET.SEEN feature is
disabled by the ~AUTO.SET.SEEN feature.
Rice [Page 41]
RFC 1203 IMAP3 February 1991
Required Features:
AUTO.SET.SEEN - When this features is enabled (default is disabled),
the \\SEEN flag is set for all appropriate messages as a side
effect of any of the following:
FETCH of RFC822
FETCH of RFC822.TEXT
COPY
Justification: This feature is provided for the use of clients
that are unable to pipeline their commands effectively and
communicate over high latency connections. When disabled,
the server will not perform any such side effects. This feature
is also provided so as to smooth the transition from IMAP2 to
IMAP3.
TAGGED.SOLICITED - When this feature is enabled (default is enabled
for IMAP3, disabled for IMAP2 mode), solicited responses from
the server will have the tag specified by the client.
When this feature is disabled, solicited responses from the
server will have the IMAP2 compatible tag "*", not the
tag specified by the client.
Justification: This feature is provided so as to smooth the
transition from IMAP2 to IMAP3.
Recommended Features.
EIGHT.BIT.TRANSPARENT - When this feature is enabled
(default is disabled), the server allows the transparent
transmission of eight bit characters. When this feature is
disabled, the value of any bit other than the least significant
7 bits transmitted by the server is unspecified. If this
feature is enabled, the characters that compose all command
keywords specified in the IMAP3 grammar and all feature names
use only their 7 least significant bits.
Justification: This feature is provided for the purpose of
supporting national character sets within messages, encoded
languages such as Japanese Kanji characters and also of binary
data, such as programs, graphics and sound.
NEW.MAIL.NOTIFY - When this feature is enabled (default is
disabled for compatibility with the majority of existing
IMAP2 servers), the server will notify the client of the
arrival of new mail in the currently selected mailbox
using the appropriate RECENT and EXISTS unsolicited messages
without the client needing to send periodic CHECK commands.
Justification: This feature is provided to allow clients to
Rice [Page 42]
RFC 1203 IMAP3 February 1991
switch off any periodic polling strategy that they may use
to look for new mail. Such polling unnecessarily uses bandwidth
and can cause the interactive performance to degrade because
the user can be kept waiting while some background process
is doing a CHECK.
SEND - When this feature is enabled (default is disabled) a new
"SEND" command becomes available to the client. The SEND
command instructs the server to send a message, rather
than requiring the client to use its own, local message
sending capability, for example. An example of of the
send command might be as follows:
tag42 SEND RFC822 {2083}
From: James Rice <Rice@Sumex-Aim.Stanford.Edu>
To:.....
If the server is unable to parse the message being sent then
it is required to issue a suitable NO notification to the client.
If the message cannot be delivered for some reason then the
server should send a suitable message to the FROM: address
of the message detailing the delivery failure.
When the SEND feature is enabled, the "send" production in
the grammar is added and as defined below. The "send"
request is added to the list of requests in the request
production also as shown below:
message_format ::= RFC822
request ::= tag SP (noop / login / logout / select / check /
expunge / copy / fetch / store / search /
select_version / select_features /
supported_versions / bboard / find /
read_only / read_write / flags /
set_flags / send) CRLF
send ::= SEND SP message_format SP string
Justification: This feature is provided so that mail can be
sent by the same reliable server that is used for the storage
of mail. This has, amongst others, the following benefits:
- Single process clients need not be delayed by mail
transmission.
- Mail sent by the client will have the server named as the
message's sender. This can be important because there are
a lot of mailers that erroneously cause reply mail to be
sent to the Sender, not the From or Reply-To address. Since
the client in general is not listening for mail being sent
to it directly this can cause mail to be lost.
Rice [Page 43]
RFC 1203 IMAP3 February 1991
- Clients can be written that do not have any native message
sending capability.
ADD.MESSAGE - When this feature is enabled (default is disabled)
a new "ADD.MESSAGE" command becomes available to the client.
The ADD.MESSAGE command instructs the server to add the
specified message to the designated mailbox. This command
can be thought of as being like a COPY command except in
this case the message that is put in the designated mailbox
is specified as a string, rather than as a message number to
be copied from the currently selected mailbox. An example
use of this command might be as follows:
tag42 ADD.MESSAGE OUTGOING-MAIL RFC822 {2083}
From: James Rice <Rice@Sumex-Aim.Stanford.Edu>
To:.....
This will have the effect of adding the message to the mailbox
called OUTGOING-MAIL.
If the server is unable to parse the message being added then
it is required to issue a suitable NO notification to the client.
When the ADD.MESSAGE feature is enabled, the "add_message"
production in the grammar is added and as defined below.
The "add_message" request is added to the list of requests
in the request production also as shown below:
add_message ::= ADD.MESSAGE SP mailbox SP format SP string
message_format ::= RFC822
request ::= tag SP (noop / login / logout / select / check /
expunge / copy / fetch / store / search /
select_version / select_features /
supported_versions / bboard / find /
read_only / read_write / flags / set_flags /
add_message) CRLF
Justification: This feature is provided so that clients can
easily add mail to specific mailboxes. This allows clients
to implement such behavior as outgoing mail storage (BCC)
without the need to resort to mailing to special BCC mailboxes.
RENUMBER - When this feature is enabled (default is disabled)
the RENUMBER command becomes available to the client.
The RENUMBER command will reorder the assignment of message
numbers to the messages in the mailbox. If this results in a
change to the association of any message number with any
message then the server is required to send solicited RESET
Rice [Page 44]
RFC 1203 IMAP3 February 1991
responses to the client. The intent of this command is
to allow users to view mailboxes in user-meaningful order
efficiently. While the client could do the ordering,
it would be less efficient in general. Note that the
server may or may not change the actual storage of the
messages and the ordering may or may not remain in effect
after another mailbox is selected or the IMAP session is
terminated. Informally, the syntax for the RENUMBER
command is:
tag RENUMBER field_name ordering_type
this has the effect of changing the IMAP grammar to be
as follows:
ordering_type ::= DATE / NUMERIC / ALPHA
renumber ::= RENUMBER SP field_name SP ordering_type
request ::= tag SP (noop / login / logout / select / check /
expunge / copy / fetch / store / search /
select_version / select_features /
supported_versions / bboard / find /
read_only / read_write / flags / set_flags /
renumber) CRLF
For example:
tag42 RENUMBER FROM ALPHA
;;;RENUMBER alphabetically by the from field
tag42 RESET 10:20,49
;;;Messages 10 to 20 and 49 have changed
tag42 OK RENUMBER finished. Sequence has changed
tag43 FETCH ALL 10:20,49
;;;Client chooses to fetch the changed msgs.
To support this the RESET message is defined as follows:
*/tag RESET message_sequence
This solicited of unsolicited message from the server informs the
client that it should flush any information that it has
retained for the specified messages.
Justification: This feature is provided so that clients can
view mailboxes in an order that is convenient to the user.
This is particularly important in the context of mailboxes
that the user copies messages to from other mailboxes. This
user-controlled filing process often does not happen in any
well-defined order. Because messages in a mailbox are
Rice [Page 45]
RFC 1203 IMAP3 February 1991
implicitly ordered (usually by arrival date, though this is
not a required ordering predicate), the user can be confused
by the apparent order of messages in the mailbox. The
addition of the RENUMBER command makes it unnecessary
for the user to leave IMAP and use some other mail system to
sort mailboxes.
ENCODING - When this feature is enabled (default is disabled) a new
generic key named ENCODING is defined. The value associated
with the generic ENCODING key is a list of (tag encoding-type
options...) lists that represent the ordered, possibly encoded
body of the message. Each such list represents a segment of
the body of the message and the way in which it is encoded.
Any options that follow the encoding_type are further
qualifiers that describe the format of the segment. Each tag
is created by the server and is unique with respect to the
other tags allocated for the other elements in the ENCODING
list. The client may use the tags returned by the server as
concrete keys to access a field which is encoded using the
encoding type and options mentioned in the appropriate list.
Thus:
tag41 FETCH 196 ENCODING ; Client asks for encoding field of msg 196.
tag41 FETCH ENCODING NIL ; Server replies. This message is not encoded.
tag41 OK Fetch completed.
tag42 FETCH 197 ENCODING ; Client asks for encoding field of msg 197.
tag42 FETCH ENCODING ((G001 UUENCODE) (G002 HEX)) ; Server replies.
tag42 OK Fetch completed.
tag43 FETCH 197 G002 ; Client asks for field named G002
tag43 FETCH G002 "A0 00 FF 13 42......." ; Server sends value of field.
tag43 OK Fetch completed.
or
tag44 STORE 197 G002 "0A 00 FF 31 24......."
; Store back the segment with nibbles swapped
Note: As a side-effect of enabling this feature, the generic key
TEXT will be redefined so as to return only those body parts of a
message that are of type TEXT. The concrete key RFC822.TEXT, on
the other hand, would still return everything in the body of the
message, even if it was full of strange, binary character
sequences.
When the client STOREs to a field denoted by one of the above tags
the server will interpret the value being passed as being in the
same format as is currently specified in the ENCODING field. The
Rice [Page 46]
RFC 1203 IMAP3 February 1991
server is not required to be able to reformat the data associated
with the ENCODING tags if the client STOREs a new value for the
ENCODING field. The interpretability of a message in the context
of its ENCODING field is undefined if the client side-effects that
ENCODING field, unless the client also STOREs new, reformatted
values for the fields that have had their encoding changed.
If the client stores a new value for the ENCODING field then the
tags in the new value will be used to index the parts of the body.
All tags in a client-STOREd ENCODING that are the same as those
originally generated by the server in response to a FETCH ENCODING
command are said still to denote the fields that they originally
denoted, though possibly reordered. Any tags not originally
defined by the server will denote new message parts, in the
appropriate format, in the relative position specified. The
exclusion of any tags that the server originally defined in a
FETCH of the ENCODING field will indicate the deletion of that
part of the message. Newly created message parts are undefined by
default, so if the client fails to follow the STOREing of the
ENCODING field with suitable STORE commands for the values
associated with any newly created tags, these fields will contain
the null value NIL.
Justification: This feature is supplied so as to allow support
for emergent multi-part and multi-media mail standards.
INDEXABLE.FIELDS - When this feature is enabled (default is
disabled) the grammar of fetch commands is changed to allow the
client to select a specific subsequence from the field in
question. For example:
tag42 FETCH 197 BODY 2000:3999
would fetch the second two thousand bytes of the body of message
197. This feature allows resource limited clients to access
small parts of large messages. The formal syntax for this is:
fetch_att ::= "ENVELOPE" / "FLAGS" / "INTERNALDATE" /
fetch_key / (fetch_key SP NUMBER ":" NUMBER)
fetch_key ::= "RFC822" / "RFC822.HEADER" / "RFC822.SIZE" /
"RFC822.TEXT" / key
If the lower bound number (the number to the left of the colon)
exceeds the maximum size of the field then the empty string is
returned. If the upper bound exceeds the maximum size of the
field but the lower bound does not then the server will return the
remaining substring of the field after the lower bound. The
Rice [Page 47]
RFC 1203 IMAP3 February 1991
bounds specified are zero indexed into the fields and the bounds
index fields by 8-bit bytes.
Justification: This feature is provided so as to allow resource-
limited clients to read very large messages and also to allow
clients to improve interactive response for the reading of large
messages by fetching the first "screen full" of data to display
immediately and fetching the rest of the message in the
background.
SET.EOL - When enabled (default is disabled), this feature
allows the new command SET.EOL to be available, changing the
grammar as follows:
character ::= "CR" / "LF" / number
request ::= tag SP (noop / login / logout / select / check /
expunge / copy / fetch / store / search /
select_version / select_features /
supported_versions / bboard / find /
read_only / read_write / flags / set_flags /
set_eol) CRLF
set_eol ::= "SET.EOL" 1#character
This has the effect of changing the end of line character sequence
generated by the server for newlines within strings to the
sequence of characters specified. The characters in the sequence
can be either the specified symbolically named characters or a
numerical value, specifying the decimal value of the character to
use. Thus, if the client would like newlines in strings to be
indicated by a carriage return followed by a control-d, the client
would issue the following command:
tag42 SET.EOL CR 4
If the server is unable to support the combination of characters
requested by the client as its end-of-line pattern it will reply
with a NO response. This might be the case, for example, if a
server is only able to generate its own native line feed pattern
and the CRLF required by IMAP by default.
The server is required to change any length denoting values, such
as envelope byte counts for all future transactions to reflect the
new eol setting. This change in reported sizes should apply to
all generic size fetching keys, but not to concrete ones such as
RFC822.SIZE, which by their very nature require a size measurement
in RFC822 format, i.e., with CRLF as the end-of-line convention.
Rice [Page 48]
RFC 1203 IMAP3 February 1991
Justification: This feature is provided because frequently clients
and servers might have end-of-line conventions other than the CRLF
specified by RFC822. It is undesirable that the IMAP be linked
too closely to RFC822 and selecting a different convention might
allow substantial performance improvements in both clients and
servers by saving either client, server or both from having to
shuffle text around so as to add or remove non-local end-of-line
sequences.
Acknowledgements:
This text is based on RFC 1064 by Mark Crispin.
The following have made major contributions to this proposed update
to the IMAP2 protocol:
James Rice <Rice@sumex-aim.stanford.edu>
Richard Acuff <acuff@sumex-aim.stanford.edu>
Bill Yeager <yeager@sumex-aim.stanford.edu>
Christopher Lane <lane@sumex-aim.stanford.edu>
Bjorn Victor <Bjorn.Victor@docs.uu.se>
Additional input was also received from:
Andrew Sweer <sweer@sumex-aim.stanford.edu>
Tom Gruber <Gruber@sumex-aim.stanford.edu>
Kevin Brock <Brock@Sumex-Aim.Stanford.Edu>
Mark Crispin <MRC@cac.washington.edu>
Security Considerations
Security issues are not discussed in this memo.
Author's Address
James Rice
Stanford University
Knowledge Systems Laboratory
701 Welch Road
Building C
Palo Alto, CA 94304
Phone: (415) 723-8405
EMail: RICE@SUMEX-AIM.STANFORD.EDU
Rice [Page 49]