Network Working Group Mark Crispin
Internet Draft: IMAP4 University of Washington
Obsoletes: RFC 1176, 1064, draft-ietf-imap-imap2bis-**.txt February 1994
Document: internet-drafts/draft-ietf-imap-imap4-00.txt
INTERNET MESSAGE ACCESS PROTOCOL - VERSION 4
Status of this Memo
This document is an Internet Draft. Internet Drafts are working
documents of the Internet Engineering Task Force (IETF), its Areas,
and its Working Groups. Note that other groups may also distribute
working documents as Internet Drafts.
Internet Drafts are draft documents valid for a maximum of six
months. Internet Drafts may be updated, replaced, or obsoleted by
other documents at any time. It is not appropriate to use Internet
Drafts as reference material or to cite them other than as a "working
draft" or "work in progress". Please check the I-D abstract listing
contained in each Internet Draft directory to learn the current
status of any this or any other Internet Draft.
This is a draft document of the IETF IMAP Working Group. It is a
preliminary specification of the IMAP4 protocol, based upon the
IMAP2bis Internet drafts, unpublished IMAP2bis.TXT document, RFC
1176, and RFC 1064. Proposed extensions to the IMAP4 protocol still
under discussion are not documented here.
A revised version of this draft document will be submitted to the RFC
editor as a Proposed Standard for the Internet Community. Discussion
and suggestions for improvement are requested. This document will
expire before 31 July 1994. Distribution of this draft is unlimited.
Comments are solicited and should be sent to imap@CAC.Washington.EDU.
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Introduction
The Internet Message Access Protocol, Version 4 (IMAP4) allows a
client to access and manipulate electronic mail messages on a server.
IMAP4 is designed to permit manipulations of remote message folders,
called "mailboxes", as if they were local. IMAP4 includes operations
for creating, deleting, and renaming mailboxes; checking for new
messages; permanently removing messages; setting and clearing flags;
RFC 822 and MIME parsing; searching; and selective fetching of
message attributes, texts, and portions thereof.
IMAP4 does not specify a means of posting mail; this function is
handled by a mail transfer protocol such as SMTP (RFC 821).
IMAP4 is designed to be upwards compatible from the IMAP2 protocol
described in RFC 1176.
IMAP4 assumes a reliable data stream such as provided by TCP. When
TCP is used, an IMAP4 server listens on port 143.
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System Model and Philosophy
There are three fundamental models of client/server email: offline,
online, and disconnected use. IMAP4 can be used in any one of these
three models.
The offline model is the most familiar form of client/server email
today, and is used by protocols such as POP-3 (RFC 1225) and UUCP.
In this model, a client application periodically connects to a
server. It downloads all the pending messages to the client machine
and deletes these from the server. Thereafter, all mail processing
is local to the client. This model is store-and-forward; it moves
mail on demand from an intermediate server (maildrop) to a single
destination machine.
The online model is most commonly used with remote filesystem
protocols such as NFS. In this model, a client application
manipulates mailbox data on a server machine. A connection to the
server is maintained throughout the session. No mailbox data are
kept on the client; the client retrieves data from the server as is
needed. IMAP4 introduces a form of the online model that requires
considerably less network bandwidth than a remote filesystem
protocol, and provides the opportunity for using the server for CPU
or I/O intensive functions such as parsing and searching.
The disconnected use model is a hybrid of the offline and online
models, and is used by protocols such as PCMAIL (RFC 1056). In this
model, a client user downloads some set of messages from the server,
manipulates them offline, then at some later time uploads the
changes. The server remains the authoritative repository of the
messages. The problems of synchronization (particularly when
multiple clients are involved) are handled through the means of
unique identifiers for each message.
Each of these models have their own strengths and weaknesses:
Feature Offline Online Disc
------- ------- ------ ----
Can use multiple clients NO YES YES
Minimum use of server connect time YES NO YES
Minimum use of server resources YES NO NO
Minimum use of client disk resources NO YES NO
Multiple remote mailboxes NO YES YES
Fast startup NO YES NO
Mail processing when not online YES NO YES
Although IMAP4 has its origins as a protocol designed to accommodate
the online model, it can support the other two models as well. This
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makes possible the creation of clients that can be used in any of the
three models. For example, a user may wish to switch between the
online and disconnected models on a regular basis (e.g. owing to
travel).
IMAP4 is designed to transmit message data on demand, and to provide
the facilities necessary for a client to decide what data it needs at
any particular time. There is generally no need to do a wholesale
transfer of an entire mailbox or even of the complete text of a
message. This makes a difference in situations where the mailbox is
large, or when the link to the server is slow.
More specifically, IMAP4 supports server-based RFC 822 and MIME
processing. With this information, it is possible for a client to
determine in advance whether it wishes to retrieve a particular
message or part of a message. For example, a user connected to an
IMAP4 server via a dialup link can determine that a message has a
2000 byte text segment and a 40 megabyte video segment, and elect to
fetch only the text segment.
In IMAP4, the client/server relationship lasts only for the duration
of the TCP connection, and mailbox state is maintained on the server.
There is no registration of clients. Except for any unique
identifiers used in disconnected use operation, the client initially
has no knowledge of mailbox state and learns it from the IMAP4 server
when a mailbox is selected. This initial transfer is minimal; the
client requests additional state data as it needs.
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Protocol Overview
The IMAP4 protocol consists of a sequence of operations consisisting
of a client command, server data, and a server command result
response. Unlike most Internet protocols, both client commands and
server command result 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 server command result
response is given the same tag as the client command.
Data and non-result responses from the server are identified by the
special reserved tag of "*". These are conventionally referred to as
"untagged data" or "unsolicited responses". This reflects the fact
that the server may send an arbitrary amount of "untagged data" in
addition to any data specifically requested in the current command.
There is no syntactic difference between data responses which result
from a specific command and those which are sent unilaterally by the
server.
One form of unilaterally transmitted untagged data that commonly
occurs is an alert of a change to the mailbox made by some process
other than the IMAP4 client or server; for example, changes in the
size of the mailbox (new mail) or in the status of individual
messages. An example of a unilaterally transmitted non-result
response is an announcement of an impending server shutdown.
There is another special reserved tag, "+", discussed below.
The server must be listening for a connection. When a connection is
opened the server sends a greeting message and then waits for
commands. This greeting is either a PREAUTH (meaning that the user
has already been identified and authenticated by an external
mechanism such as rsh) or OK (meaning that the user is not yet
authenticated) unsolicited response. The server may also send a BYE
unsolicited response and close the connection if it rejects the
connection.
The client opens a connection and waits for the greeting. The client
must not send any commands until it has received the greeting from
the server.
Once the greeting has been received, the client may begin sending
commands. It is not under any obligation to wait for a server
response to a command before sending another command, subject to the
constraints of underlying flow control. When commands are received
the server acts on them and responds with command result responses,
often prefixed by some amount of server untagged data.
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In general, the command result responses do not themselves contain
the requested data. Instead, they indicate the completion status of
the request. There are three fundamental command result responses:
success (OK), error (NO), request faulty or not understood (BAD).
The effect of a command can not be considered complete until a
command result response with a tag matching the command is received
from the server.
It is not required that a server process a command to completion
before beginning processing of the next command, except when the
processing of the previous command may affect the results of the next
command by changing the state of the current mailbox. This rule has
certain other effects; for example, this implies that an EXPUNGE
response can not be transmitted as part of a response to a command
that uses sequence numbers, because EXPUNGE results in message
numbers being changed.
Client implementations should update their local cache of data with
any untagged data received from the server. Unlike command result
responses, data are not necessarily associated with specific
commands. The tagged command result response signals that the client
cache is now updated with the results of the corresponding command.
One artifact of this rule is the way new mail notification from the
server to the client takes place. If, during the execution of any
command, the server observes that the mailbox size has changed, the
server should output an unsolicited EXISTS and RECENT response
reflecting the changed size to alert the client. Similarly, any
observed change in message status should cause an unsolicited FETCH
response with the new flag data.
If authentication has not yet been completed, it must now be done via
the LOGIN command before any access to data is permitted. The only
permitted commands before successful authentication are LOGIN, NOOP,
and LOGOUT. See the section below on authentication.
Once authenticated, the client must send a mailbox selection command
to access the desired mailbox. No mailbox is selected by default.
The format of a mailbox name is implementation-dependent. However,
the mailbox name INBOX is a special name reserved to mean "the
primary mailbox for this user on this server". If it is desired to
export hierarchical mailbox names, mailbox names must be left-to-
right hierarchical using a single character to separate levels of
hierarchy. The same hierarchy separator character is used for all
levels of hierarchy within a single name.
By convention, if the name of the first hierarchical element of a
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mailbox name starts with the character "#", the element is considered
a namespace. A namespace should not be presented as part of the
proper name of a mailbox, but rather as the name of a well-known
collection of mailboxes. For example, some IMAP implementations may
wish to partition netnews mailboxes from other mailboxes using a
"news" namespace.
On a successful selection, the server will send a list of valid
flags, number of messages, and number of messages arrived since last
access for this mailbox as untagged data, followed by an OK command
result response. The client may close access to this mailbox and
access a different one with another selection command.
Several flags are predefined in IMAP4. All predefined IMAP4 flags
(called, for historical reasons, "system flags") begin with a
backslash ("\") character. Servers MUST, at a minimum, support all
the predefined flags in this specification. In addition, a server
may also have some implementation-defined per-mailbox flags (called,
for historical reasons, "keywords") that do not begin with backslash.
Clients should use the information from the server's FLAGS response
at message selection time to determine what flags the server supports
for the currently selected mailbox.
The client requests mailbox data with FETCH commands, and receives it
via the untagged data mechanism. Three major categories of mailbox
data exist.
The first category is data that are associated with a message as an
entity in the mailbox. There are now four such items of data: the
"internal date", the "RFC 822 size", the "flags", and the "unique
id". The internal date is the date and time that the message was
received by the server. The RFC 822 size is the size in octets of
the message, expressed as an RFC 822 text string. The flags are a
list of status flags associated with the message. The unique id is
an identifier that is guaranteed to refer to this message and to none
other in the mailbox and that, unlike IMAP4 sequence numbers,
persists across sessions and never changes as a result of an EXPUNGE.
The second category is data that describe the composition and
delivery information of a message; that is, information such as the
message sender, recipient lists, message-ID, subject, MIME structure,
etc. This is the information that is stored in the RFC 822 and MIME
headers. In IMAP4, the RFC 822 header information that may be
fetched is called the "envelope structure" (not to be confused with
SMTP (RFC 821) envelopes). Similarly, the MIME header information
that may be fetched is called the "body structure". A client can use
the parsed envelope and body structures and not worry about having to
do its own RFC 822 or MIME parsing.
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The third category is textual data, some of which are intended for
direct human viewing. IMAP4 defines six such items: RFC822.HEADER,
RFC822.HEADER.LINES, RFC822.HEADER.LINES.NOT, RFC822.TEXT, RFC822,
and MIME body parts. It is possible to fetch an individual MIME body
part of a message without fetching any other data associated with the
message.
A simple client can "FETCH RFC822" to get the entire message without
any processing. A more advanced client might fetch some combination
of the first and second categories of data for use as a presentation
menu. Then, when the user wishes to read a particular message, it
will fetch the appropriate texts.
Individual data items in IMAP4 are represented as atoms (one or more
non-special characters), strings (a quoted sequence of characters),
or numbers. Some data items may be either atoms or strings.
Data structures in IMAP4 are represented as an S-expression list
similar to that used in the Lisp programming language. An S-
expression consists of a sequence of data items delimited by space
and bounded at each end by parentheses. An S-expression may itself
contain other S-expressions, using parentheses to indicate nesting.
S-expression syntax was chosen because it provides a concise and
precise means of expressing nested data (e.g. MIME structures).
The client can alter certain data with a STORE command. As an
example, a message is marked for deletion from a mailbox by setting
the \Deleted flag with a STORE command.
Other client operations that can be done to a mailbox include copying
messages to other mailboxes, permanently removing messages that are
marked as deleted, checking for updated mailbox state, and searching
for messages that match certain criteria. It is also possible to
select a different mailbox, create a new mailbox, rename an existing
mailbox, or delete an existing mailbox.
The client should end the session with the LOGOUT command. The
server returns a "BYE" followed by an "OK", at which point both the
client and the server close the connection. If the client closes the
network connection without a LOGOUT command, the server should do its
normal logout procedures without attempting any further interaction
with the client.
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Authentication
Pre-authentication is only possible when the connection to the IMAP4
service is made through some link protocol that provides its own
authentication mechanism. It is not used with a TCP connection to
port 143.
An example of pre-authentication is the BSD "RSH" protocol, that
provides authentication through a "trusted host" facility. Another
example would be a manual invocation of an IMAP4 server from a
logged-in timesharing job.
A pre-authenticated IMAP4 server should recognize that authentication
has already happened, and enter the post-login state. In its
greeting message, it should use the unsolicited response "PREAUTH"
instead of "OK" to indicate that external authentication has taken
place.
This is an example of a pre-authentication scenario. In this and all
other examples in this document, S: indicates server dialog and C:
indicates client dialog.
S: * PREAUTH IMAP4 Server pre-authenticated as user "Smith"
C: A001 SELECT INBOX
S: * FLAGS (\Answered \Flagged \Deleted \Seen \Junked)
S: * 19 EXISTS
S: * 2 RECENT
S: * OK [UNSEEN 14] Message 14 is the first unseen message
S: A001 OK SELECT complete
A connection that is not pre-authenticated is constrained to using
the LOGIN command for establishing authentication. Authentication
via the LOGIN command is with either a user name and password pair,
or with a user identifier and Kerberos authenticator. See the
description of the LOGIN command for more details.
Servers may allow non-authenticated access to certain mailboxes. The
convention is to use a LOGIN command with the userid "anonymous". A
password is still required. It is implementation-dependent what
requirements, if any, are placed on the password and what access
restrictions are placed on anonymous users.
Implementations are NOT required to support pre-authentication,
Kerberos authentication, or the anonymous convention.
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Summary of Defined Commands and Responses
Commands || Responses
-------- || ---------
||
Pre-Authentication Commands || Tagged Command Result Responses
--------------------------- || -------------------------------
NOOP / LOGIN / LOGOUT || OK / NO / BAD
||
Mailbox Management || Untagged Status Responses
------------------ || -------------------------
CREATE / DELETE / RENAME || PREAUTH / OK / NO / BAD / BYE
LIST / LSUB / SUBSCRIBE / ||
UNSUBSCRIBE || Untagged Listing Responses
|| --------------------------
Mailbox Selection || LIST / LSUB
----------------- ||
SELECT / EXAMINE || Untagged Data Responses
|| -----------------------
Message Management || FLAGS / SEARCH
------------------ || EXISTS / RECENT / EXPUNGE
CHECK / EXPUNGE || FETCH
||
Message Copying || Command Continuation Ready Response
--------------- ||
APPEND / COPY ||
||
Message Data Transfer ||
FETCH / PARTIAL / STORE / ||
SEARCH / UID / x_command ||
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Commands
tag NOOP
The NOOP command returns an OK to the client. By itself, it does
nothing else. However, since any command can return a status
update as untagged data, this command can be used to poll for new
mail or for message status updates.
Another possible use of this command is for the client to "ping"
the server so that the client and server know that each other are
still alive. This is useful with servers that have an inactivity
autologout timer.
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: C: a001 LOGIN SMITH SESAME
S: a001 OK LOGIN completed
logs in as user SMITH with password SESAME.
If a server supports authentication via Kerberos, it may accept
the string "@KERBEROS:" followed by the hexadecimal representation
of a Kerberos authenticator.
EXAMPLE: The following is a Kerberos login scenario (note that the
line breaks in the sample authenticator are for editorial clarity
and are not in a real authenticator):
S: * OK Kerberos IMAP4 Server
C: a001 LOGIN smith @KERBEROS:040700414e445245572e434d552e4544550
038202c868f3890b377fc8266acc1bedb96b80d3fa76489898e74cd1c952dc
4003ea3428f29f1c470016cf5adc22f939e6deff2747254c1815d5b0b90d4c
5a2cba21eb0abe32f9acbf568d751bf4cc13f5ba4e6d82c638a8b5421
S: a001 OK [df84a4cb8323454f] Login OK via Kerberos
The token in the brackets in the OK response is the Kerberos
authentication response, encrypted with the session key in network
byte order and an incremented checksum as in the usual Kerberos
procedure.
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tag LOGOUT
The LOGOUT command informs the server that the client is done with
the session. The server should send an unsolicited BYE response
before the (tagged) OK response, and then close the network
connection.
EXAMPLE: C: A023 LOGOUT
S: * BYE IMAP4 Server logging out
S: A023 OK LOGOUT completed
Server and client then close the connection.
Mailbox manipulation commands - CREATE, DELETE, RENAME
These commands permit the manipulation of entire mailboxes.
tag CREATE mailbox
The CREATE command creates a mailbox with the given name. This
command returns an OK response only if a new mailbox with that
name has been created. It is an error to attempt to create a
mailbox with a name that refers to an extant mailbox. Any
error in creation will return a NO response.
If the mailbox argument is suffixed with the hierarchy
separator character (obtained from the server via a LIST
command), this is a declaration to the server that the client
may, in the future, create mailbox names under this name in the
hierarchy. Server implementations that do not require this
declaration should ignore it.
Discussion: The IMAP4 protocol permits the selection
of non-terminal names in the hierarchy as mailboxes,
and the creation of new names under a formerly
terminal name in the hierarchy. In some server
implementations, it may not be possible for a name
to be both a level of hierarchy and a selectable
mailbox, or only certain classes of mailbox may be
at non-terminal levels of hierarchy. The hierarchy
separator character suffix provides the necessary
hint for the server to choose which type of name to
create.
Creating INBOX is not permitted.
EXAMPLES: C: a003 CREATE blurdybloop
S: a003 OK CREATE completed
creates a mailbox named "blurdybloop".
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C: a004 CREATE owatagusiam/
S: a004 OK CREATE completed
C: a005 CREATE owatagusiam/blurdybloop
S: a005 OK CREATE completed
where "/" was returned as the hierarchy separator from LIST,
creates a new level of hierarchy named "owatagusiam", and then
creates a mailbox named "blurdybloop" inside "owatagusiam".
tag DELETE mailbox
The DELETE command deletes a mailbox with the given name. This
command returns an OK response only if a mailbox with that name
has been deleted. It is an error to attempt to delete a
mailbox name that does not exist. Any error in deletion will
return a NO response.
A server SHOULD NOT attempt to test that a mailbox is empty
before permitting deletion; this would prevent the deletion of
a mailbox that for some reason can not be opened or expunged,
leaving to possible denial of service problems. Any such
checking should be left to the discretion of the client.
Deleting INBOX is not permitted.
EXAMPLE: C: a003 DELETE blurdybloop
S: a003 OK DELETE completed
deletes the mailbox named "blurdybloop".
tag RENAME old_mailbox new_mailbox
The RENAME command changes the name of a mailbox. This command
returns an OK response only if a mailbox with the old name
exists and has been successfully renamed to the new name. It
is an error to attempt to rename with an old mailbox name that
does not exist or a new mailbox name that already exists. Any
error in renaming will return a NO response.
Renaming INBOX is permitted. A new, empty INBOX is created in
its place.
EXAMPLE: C: a003 RENAME blurdybloop owatagusiam
S: a003 OK RENAME completed
renames the mailbox named "blurdybloop" to the new name
"owatagusiam".
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Listing commands - LIST, LSUB
The listing commands accept two arguments, a reference and a mailbox
name. These commands return some set of unsolicited replies that
have as their value a list of attributes, a separator character, and
a mailbox name.
The reference is a default name which may affect the interpretation
of the mailbox name in an implementation-dependent fashion. The
purpose of the reference is to provide a context in which the mailbox
name is interpreted. For example, given a UNIX-based server and a
reference of "~smith/Mail/", the mailbox "foo" results in
"~smith/Mail/foo". However, the name "/usr/bboard/foo" with the same
reference results in "/usr/bboard/foo" since the leading "/"
overrides the current working directory on UNIX.
Three wildcard characters are defined in the mailbox argument:
* zero or more characters may match at this position
% zero or more characters, except for a hierarchy delimiter,
may match at this position
? any single character (including a hierarchy delimiter) may
match at this position
Two name attributes are presently defined. \Noinferiors indicates
that it is not possible for any child levels of hierarchy to exist
under this name (that is, no child levels exist now and none can be
created in the future). \Noselect indicates it is not possible to
use this name as a selectable mailbox.
The separator character delimits levels of hierarchy in a mailbox
name. A client may use it to create child mailboxes, and to search
higher or lower levels of naming hierarchy. All children of a top-
level hierarchy node must use the same separator character. A NIL
separator means that no hierarchy exists; the name is a "flat" name.
The returned mailbox names are canonicalized to a simple left-to-
right hierarchy with a single separator character. If the server
does not support hierarchy, then the return named are in a form
matching the reference (if appropriate) and name used in the request.
The returned mailbox name MUST be valid as a new reference; and,
unless \Noselect is indicated, it must also be valid as an argument
for commands which accept a mailbox name such as SELECT.
For example, on a server in which "." is the hierarchy delimiter, and
where the set of valid names are abc, bar, foobar, foodbar, foo.bar,
and food.on.the.bar, the results from various references and patterns
are:
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Reference Pattern Results
--------- ------- -------
* abc, bar, foobar, foodbar, foo.bar,
food.on.the.bar
% abc, bar, (\Noselect) foo, (\Noselect) food,
foobar, foodbar
*bar bar, foobar, foodbar, foo.bar, food.on.the.bar
%bar bar, foobar, foodbar
foo?bar foodbar, foo.bar
foo*bar foobar, foodbar, foo.bar, food.on.the.bar
foo *bar foobar, foodbar, foo.bar, food.on.the.bar
foo. *bar foo.bar
Depending upon the implementation, the \Noinferiors attribute may
also be associated with the names bar, foobar, foodbar, foo.bar, and
food.on.the.bar.
tag LIST reference mailbox
The LIST command accepts as arguments a reference name followed
by a mailbox name, including possible wildcards, that specifies
some subset of names from the complete set of all names
available to the user. Zero or more unsolicted LIST replies
are returned.
The special name INBOX is included in the output from LIST
unless INBOX is not supported by this server or for this user.
The criteria for omitting INBOX is whether SELECT INBOX will
return failure; it is not relevant whether the user's real
INBOX resides on this or some other server. LIST and LSUB
provide a mechanism for the user to identify that this is his
or her real INBOX.
The exact meaning of "names available to the user" is
implementation-dependent, since the concept of a bounded or
deterministic set of "mailboxes available to the user" may not
be meaningful for a particular server or server implementation.
EXAMPLE: C: A002 LIST "~/Mail/" "%"
S: * LIST (\Noselect) "/" ~/Mail/foo
S: * LIST () "/" ~/Mail/meetings
S: A002 OK LIST completed
tag LSUB reference mailbox
The LSUB command accepts as arguments reference name followed
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by a mailbox name, including possible wildcards, that specifies
some subset of names from the set of names that the user has
declared as being "active" or "subscribed". Zero or more
unsolicited LSUB replies are returned.
The exact meaning of "subscribed" is implementation-dependent,
since the concept of a set of "subscribed" names that is
preserved across sessions may not be meaningful for a
particular server or server implementation.
EXAMPLE: C: A002 LSUB "#news." "comp.mail.*"
S: * LSUB () "." #news.comp.mail.mime
S: * LSUB () "." #news.comp.mail.misc
S: A002 OK LSUB completed
indicates that the user is subscribed to the two news groups
listed, and to no other news groups with names beginning with
"comp.mail.".
Subscription commands - SUBSCRIBE, UNSUBSCRIBE
These commands permit the manipulation of mailbox subscriptions.
Subscription status should be preserved between sessions.
tag SUBSCRIBE mailbox
The SUBSCRIBE command adds the specified mailbox name to the
list of "active" or "subscribed" mailboxes as returned by the
LSUB command. This command returns an OK response only if the
subscription is successful.
EXAMPLE: C: A002 SUBSCRIBE comp.mail.mime
S: A002 OK SUBSCRIBE completed
tag UNSUBSCRIBE mailbox
The UNSUBSCRIBE command removes the specified mailbox name from
the list of "active" or "subscribed" mailboxes as returned by
the LSUB command. This command returns an OK response only if
the unsubscription is successful.
EXAMPLE: C: A002 UNSUBSCRIBE comp.mail.mime
S: A002 OK UNSUBSCRIBE completed
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tag SELECT mailbox
This command selects a particular mailbox. The server must check
that the user is permitted read access to this mailbox. Before
returning an OK to the client, the server MUST send the following
untagged data to the client:
FLAGS mailbox's defined flags
<n> EXISTS the number of messages in the mailbox
<n> RECENT the number of messages added to the mailbox since the
previous time this mailbox was read
to define the initial state of the mailbox at the client. If it
can not be determined which messages were added since the previous
time a mailbox was read, then all messages SHOULD be considered
recent. An example of this is if no "last read" time information
is available or a read-only mailbox that does not permit a change
of "last read" time.
The server SHOULD also send an UNSEEN special information token in
an unsolicited OK, indicating the first unseen message in the
mailbox.
If the client may not change the permanent state of one or more of
the flags listed in the FLAGS unsolicited response, the server
SHOULD send a PERMANENTFLAGS special information token in an
unsolicited OK, listing the flags that the client may change
permanently. Note that the PERMANENTFLAGS special information
token can also be used to inform the client that it is possible to
define new user flags.
Multiple selection commands are permitted in a session. The
previous mailbox is automatically deselected when a new selection
is made. If concurrent access to multiple mailboxes is desired,
the client should open additional sessions as needed.
The text of an OK response to the SELECT command should begin with
either the "[READ-ONLY]" or "[READ-WRITE]" special information
token to show the mailbox's access status. Other special
information tokens may be defined in a revision to this protocol.
EXAMPLE: C: A002 SELECT INBOX
S: * 172 EXISTS
S: * 1 RECENT
S: * OK [UNSEEN 12] Message 12 is the first unseen message
S: * FLAGS (\Answered \Flagged \Deleted \Seen \Junked)
S: A002 OK [READ-WRITE] SELECT completed
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tag EXAMINE mailbox
The EXAMINE command is similar to SELECT, and returns the same
output; however, the selected mailbox is identified as read-only
and no changes are permitted to this mailbox.
EXAMPLE: C: A002 EXAMINE blurdybloop
S: * 17 EXISTS
S: * 2 RECENT
S: * OK [UNSEEN 8] Message 8 is the first unseen message
S: * FLAGS (\Answered \Flagged \Deleted \Seen \Junked)
S: A002 OK [READ-ONLY] EXAMINE completed
tag CHECK
The CHECK command requests a checkpoint of the mailbox. CHECK may
cause an operation that may take a non-instantaneous amount of
real-time to complete. The exact meaning of a checkpoint is
implementation-dependent. Possible interpretations include
forcing an update of the server's disk of all changes made to the
selected mailbox, rescanning of the entire mailbox, etc. If an
implementation has no such considerations, CHECK should be
equivalent to NOOP.
CHECK should NOT be used to poll for new mail; new mail checking
happens implicitly as part of every command. NOOP should be used
for any new mail polling. CHECK should NOT be used to get the
current size of the mailbox; there is no guarantee that CHECK will
cause an EXISTS or RECENT unsolicited response.
EXAMPLE: C: a001 CHECK
S: a001 OK CHECK completed
tag EXPUNGE
The EXPUNGE command permanently removes all messages with the
\Deleted flag set from the currently selected mailbox. Before
returning an OK to the client, for each message that is removed,
an unsolicited EXPUNGE response is sent. The message number for
each successive message in the mailbox is immediately decremented
by 1; this means that if the last 5 messages in a 9-message mail
file are expunged the client will receive 5 unsolicited EXPUNGE
responses for message 5.
EXAMPLE: C: A007 EXPUNGE
S: 3 EXPUNGE
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S: 5 EXPUNGE
S: 8 EXPUNGE
S: A007 EXPUNGE completed
In this example, messages 3, 6, and 10 had the \Deleted flag set.
After message 3 was expunged, message 6 became message 5, and
message 10 became message 9. After message 5 was expunged,
message 9 became message 8.
tag COPY sequence mailbox
The COPY command copies the specified message(s) to the specified
destination mailbox. The flags and internal date of the
message(s) SHOULD be preserved in the copy.
If the destination mailbox does not exist, a server SHOULD return
an error. It SHOULD NOT automatically create the mailbox. Unless
it is certain that the destination mailbox can not be created, the
server MUST send the special information token "[TRYCREATE]" as
the prefix of the text of the tagged NO response. This gives a
hint to the client that it can attempt a CREATE command and retry
the COPY if the CREATE is successful.
If the COPY command is unsuccessful for any reason, IMAP4 server
implementations MUST restore the destination mailbox to its prior
state before the COPY attempt.
EXAMPLE: C: A003 COPY 2:4 MEETING
S: A003 COPY completed
copies messages 2, 3, and 4 to mailbox "MEETING".
tag APPEND mailbox [flag_list] [date_time] literal
The APPEND command appends the literal argument as a new message
in the specified destination mailbox. This argument is in the
format of an RFC 822 message. If any flags or date_time are
specified, those flags and internal date SHOULD be set in the
resulting message. If the append is unsuccessful for any reason
the mailbox must be restored to its prior state before the APPEND
attempt; no partial appending is permitted. If the mailbox is
currently selected, the normal new mail actions should occur.
Server implementations SHOULD return a NO response if the length
of the literal is zero.
If the destination mailbox does not exist, a server MUST return an
error, and MUST NOT automatically create the mailbox. Unless it
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is certain that the destination mailbox can not be created, the
server MUST send the special information token "[TRYCREATE]" as
the prefix of the text of the tagged NO response. This gives a
hint to the client that it can attempt a CREATE command and retry
the APPEND if the CREATE is successful.
Note that this functionality is unsuitable for message delivery,
because it does not provide a mechanism to transfer SMTP (RFC 821)
envelope information.
EXAMPLE: C: A003 APPEND saved-messages (\Seen) {311}
C: Date: Mon, 7 Feb 1994 21:52:25 -0800 (PST)
C: From: Fred Foobar <foobar@Blurdybloop.COM>
C: Subject: afternoon meeting
C: To: mooch@owatagu.siam.edu
C: Message-Id: <B27397-0100000@Blurdybloop.COM>
C: Mime-Version: 1.0
C: Content-Type: TEXT/PLAIN; CHARSET=US-ASCII
C:
C: Hello Joe, do you think we can meet at 3:30 tommorrow?
C:
S: A003 OK APPEND completed
tag FETCH sequence data
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 currently defined data items that
can be fetched are:
ALL Macro equivalent to:
(FLAGS INTERNALDATE RFC822.SIZE ENVELOPE)
BODY Non-extensible form of BODYSTRUCTURE.
BODY[section] The text of a particular body section. The
section specification is a set of one or more
part numbers delimited by periods.
Single-part messages only have a part 1.
Multipart messages are assigned consecutive
part numbers, as they occur in the message.
If a particular part is of type message or multipart,
its parts must be indicated by a period followed by
the part number within that nested multipart part.
It is not permitted to fetch a multipart part
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itself, only its individual members.
A part of type MESSAGE and subtype RFC822 also
has nested parts. These are the parts of the
MESSAGE part's body. Nested part 0 of a part of
type MESSAGE and subtype RFC822 is the RFC 822
header of the message.
Every message has at least one part.
EXAMPLE: Here is a complex message with its
associated section specifications.
1 TEXT/PLAIN
2 APPLICATION/OCTET-STREAM
3 MESSAGE/RFC822
3.0 (RFC 822 header of the message)
3.1 TEXT/PLAIN
3.2 APPLICATION/OCTET-STREAM
[4] MULTIPART/MIXED
4.1 IMAGE/GIF
4.2 MESSAGE/RFC822
4.2.0 (RFC 822 header of the message)
4.2.1 TEXT/PLAIN
[4.2.2] MULTIPART/ALTERNATIVE
4.2.2.1 TEXT/PLAIN
4.2.2.2 TEXT/RICHTEXT
Note that there is no section specification for
the Multi-part parts (no section 4 or 4.2.2).
These non-existent section specifications may,
however, be used in the MD5[section] fetch item
(see below).
The \Seen flag is implicitly set; if this causes
the flags to change they should be included as
part of the fetch results.
BODY.PEEK[section]
An alternate form of BODY[section] that does not
implicitly set the \Seen flag.
BODYSTRUCTURE The MIME body structure of the message. This
is computed by the server by parsing the MIME
header lines.
ENVELOPE The envelope structure of the message. This is
computed by the server by parsing the RFC 822
header into the component parts, defaulting
various fields as necessary.
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FAST Macro equivalent to:
(FLAGS INTERNALDATE RFC822.SIZE)
FLAGS The flags that are set for this message.
FULL Macro equivalent to:
(FLAGS INTERNALDATE RFC822.SIZE ENVELOPE BODY)
INTERNALDATE The date and time of final delivery of the
message as defined by RFC 821.
MD5[section] The server-calculated MD5 checksum of the
particular body section in BASE64 encoding.
Unlike BODY[section], it is permitted to
fetch the MD5 checksum of a multipart section
number.
RFC822 The message in RFC 822 format. The \Seen
flag is implicitly set; if this causes the
flags to change they should be included as
part of the fetch results. This is the
concatenation of RFC822.HEADER and RFC822.TEXT.
RFC822.PEEK An alternate form of RFC822 that does not
implicitly set the \Seen flag.
RFC822.HEADER The RFC 822 format header of the message as
stored on the server including the delimiting
blank line between the header and the body.
RFC822.HEADER.LINES header_line_list
All header lines (including continuation lines)
of the RFC 822 format header of the message
with a field-name (as defined in RFC 822) that
matches any of the strings in header_line_list.
The matching is case-independent but otherwise
exact.
RFC822.HEADER.LINES.NOT header_line_list
All header lines (including continuation lines)
of the RFC 822 format header of the message
with a field-name (as defined in RFC 822) that
does not match any of the strings in
header_line_list. The matching is
case-independent but otherwise exact.
RFC822.SIZE The number of characters in the message as
expressed in RFC 822 format.
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RFC822.TEXT The text body of the message, omitting the
RFC 822 header. The \Seen flag is implicitly
set; if this causes the flags to change they
should be included as part of the fetch results.
RFC822.TEXT.PEEK
An alternate form of RFC822.TEXT that does not
implicitly set the \Seen flag.
UID The unique identifier for the message.
EXAMPLES: C: A003 FETCH 2:4 ALL
fetches the flags, internal date, RFC 822 size, and envelope
structure for messages 2, 3, and 4.
C: A004 FETCH 3 RFC822
fetches the RFC 822 representation for message 3.
C: A005 FETCH 4 (FLAGS RFC822.HEADER)
fetches the flags and RFC 822 format header for message 4.
C: A006 FETCH 3 BODY[4.1]
fetches body part 4.1 for message 3.
C: A007 FETCH 3 RFC822.HEADER.LINES (SUBJECT DATE TO CC FROM)
fetches the five listed header lines for message 3.
tag PARTIAL msgno data start_octet octet_count
The PARTIAL command is equivalent to the associated FETCH command,
with the added functionality that only the specified number of
octets, beginning at the specified starting octet, are returned.
Only a single message can be fetched at a time. The first octet
of a message, and hence the minimum for the starting octet, is
octet 1.
The following FETCH items are valid data for PARTIAL: RFC822,
RFC822.PEEK, RFC822.HEADER, RFC822.TEXT, RFC822.TEXT.PEEK,
BODY[section], and BODY.PEEK[section].
Any partial fetch that attempts to read beyond the end of the text
is truncated as appropriate. If the starting octet is beyond the
end of the text, an empty string is returned.
The data are returned with the FETCH response. There is no
indication of the range of the partial data in this response; thus
it is generally not possible to implement caching with PARTIAL
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data. It is also not possible to stream multiple PARTIAL commands
of the same data item without processing and synchronizing at each
step, since each PARTIAL fetch of data replaces any prior
(PARTIAL) FETCH of the data.
Note that a possible problem exists in partial fetching. It is
possible to break in the middle of a line or a critical sequence
such as a BASE64 quadruple or QUOTED-PRINTABLE shift.
Implementations using partial fetching should keep this in mind.
There is no requirement that partial fetches follow any sequence;
so if it turns out that a partial fetch of octets 1 through 10000
breaks in an awkward place, it is permitted to continue with a
partial fetch of 9987 through 19987, etc.
The handling of the \Seen flag is the same as in the associated
FETCH command.
EXAMPLE: C: A005 PARTIAL 4 RFC822 1 1024
S: * 1 FETCH (RFC822 {1024}
S: Return-Path: <gray@cac.washington.edu>
S: ...
S: ......... FLAGS (\Seen))
S: A005 OK PARTIAL completed
tag STORE sequence data value
The STORE command alters data associated with a message in the
mailbox. The currently defined data items that can be stored are:
FLAGS Replace the flags for the message with the
argument (in flag list format).
+FLAGS Add the flags in the argument to the
message's flag list.
-FLAGS Remove the flags in the argument from the
message's flag list.
EXAMPLE: A003 STORE 2:4 +FLAGS (\Deleted)
marks messages 2, 3, and 4 for deletion.
tag UID command argument(s)
The UID command takes as its arguments a COPY, FETCH, SEARCH, or
STORE command with arguments appropriate for the associated
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command. However, instead of message numbers, it uses unique
identifiers in the sequence argument to reference a particular
message or range of messages. The unique identifier of a message
is guaranteed not to refer to any other message in the mailbox.
Unlike IMAP4 sequence numbers, unique identifiers persist across
sessions.
Sequence ranges are permitted; note however that there is no
guarantee that unique identifiers be contiguous. A non-existent
unique identifier within a sequence range is ignored without any
error message generated.
The number after the "*" in a message data response (most notably,
FETCH) is always a message number, not a unique identifier, even
in the case of a UID command response. However, servers MUST
implicitly include UID as part of any FETCH response caused by a
UID command, regardless of whether UID was specified as a FETCH
data item.
The numbers returned in a SEARCH response are unique identifiers
in the case of a UID SEARCH.
EXAMPLE: C: A003 UID FETCH 4827313:4828442 FLAGS
S: * 23 FETCH (FLAGS (\Seen) UID 4827313)
S: * 24 FETCH (FLAGS (\Seen) UID 4827943)
S: * 25 FETCH (FLAGS (\Seen) UID 4828442)
S: A003 UID FETCH completed
Note that the UID was returned for each message, even though the
FETCH request did not explicitly specify UID as a FETCH data item.
tag SEARCH [search_charset] search_program
The SEARCH command searches the mailbox for messages that match
the given set of criteria inside a search program. The
unsolicited "SEARCH number(s)" response from the server is a list
of message numbers corresponding to those messages which match the
search program.
When multiple criteria are specified in a search program, the
result is the intersection (AND function) of all the messages that
match that criteria. For 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 mail file since October 1, 1987. Search
criteria may also be parenthesized groups of criteria, for use
with the OR and NOT criteria operators.
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Server implementations MAY exclude MIME body parts with terminal
content types other than TEXT and MESSAGE.
The optional search_charset consists of the atom "CHARSET"
followed by a registered MIME character set. It indicates the
character set of subsequent strings used in search criteria. RFC
1522 strings which appear in RFC 822/MIME message headers MUST be
decoded prior to matching. Server implementations MUST also
decode MIME content transfer encodings prior to matching. If the
server does not support the specified character set, it must
return a tagged NO response (not a BAD). However, server
implementations MUST support the character set US-ASCII.
In all search criteria that use strings, a message matches the
criteria if the string is a substring of the field. The matching
is case-independent.
The currently defined search criteria used in search programs are:
sequence Messages within the specified sequence range.
ALL All messages in the mailbox; the default
initial criterion for ANDing.
ANSWERED Messages with the \Answered flag set.
BCC astring Messages that contain the specified string
in the envelope structure's BCC field.
BEFORE date Messages whose internal date is earlier than
the specified date.
BODY astring Messages that contain the specified string
in the body of the message.
CC astring Messages that contain the specified string
in the envelope structure's CC field.
DELETED Messages with the \Deleted flag set.
FLAGGED Messages with the \Flagged flag set.
FROM astring Messages that contain the specified string
in the envelope structure's FROM field.
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HEADER header_line astring
Messages that have a header with a field-name
(as defined in RFC 822) of header_line and
which contains the specified string in the
RFC 822 field-body.
JUNKED Messages with the \Junked flag set.
KEYWORD flag Messages with the specified flag set.
LARGER number Messages larger than the specified number of
octets.
NEW Messages that have the \Recent flag set but
not the \Seen flag. This is functionally
equivalent to "RECENT UNSEEN".
NOT search_criterion
Messages that do not match the specified
search criterion.
OLD Messages that do not have the \Recent flag
set. This is functionally equivalent to
"NOT RECENT" (as opposed to "NOT NEW").
ON date Messages whose internal date is within the
specified date.
OR search_criterion1 search_criterion2
Messages which match either search criterion.
RECENT Messages that have the \Recent flag set.
SEEN Messages that have the \Seen flag set.
SENTBEFORE date Messages whose RFC 822 Date: header is
earlier than the specified date.
SENTON date Messages whose RFC 822 Date: header is
within the specified date.
SENTSINCE date Messages whose RFC 822 Date: header is
within or later than the specified date.
SINCE date Messages whose internal date is within or
later than the specified date.
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SMALLER number Messages smaller than the specified number of
octets.
SUBJECT astring Messages that contain the specified string
in the envelope structure's SUBJECT field.
TEXT astring Messages that contain the specified string in
the header or body of the message.
TO astring Messages that contain the specified string in
the envelope structure's TO field.
UID sequence Messages with unique identifiers within the
specified unique identifier sequence range.
UNANSWERED Messages that do not have the \Answered flag
set.
UNDELETED Messages that do not have the \Deleted flag
set.
UNFLAGGED Messages that do not have the \Flagged flag
set.
UNJUNKED Messages that do not have the \Junked flag
set.
UNKEYWORD flag Messages that do not have the specified flag
set.
UNSEEN Messages that do not have the \Seen flag set.
EXAMPLES: C: A003 SEARCH HEADER Keywords "Paris"
matches messages with "Paris" in the Keywords header.
C: A003 SEARCH NOT SUBJECT "Paris meeting"
matches messages which don't have "Paris meeting" in the subject header.
C: A007 SEARCH FROM SMITH (OR DELETED JUNKED)
Messages which are from Smith and have the \Deleted or \Junked flag set.
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Responses
Command Completion Responses
This group of responses indicate the completion of a command, and
whether or not that command was successful. These responses are
tagged, using the tag from the command. The response text is a line
of human readable text, optionally prefixed by data inside square
brackets that convey a special information token and possible
arguments.
The currently defined special information tokens are:
PARSE An error occurred in parsing the RFC 822 or MIME
headers of a message in the mailbox.
PERMANENTFLAGS Followed by a list of flags, indicates which of the
known flags that the client may change permanently.
The special flag \* indicates that it is possible
to create new user flags by attempting to store those
flags in the mailbox. Any flags which are in the
FLAGS unsolicited response, but not the PERMANENTFLAGS
list, can not be set permanently. An attempt to STORE
a flag that is not in the PERMANENTFLAGS list MUST
either be rejected with a NO reply or affect the state
for the remainder of the current session only.
READ-ONLY The mailbox is open read-only, or its access while
open has changed from read-write to read-only.
READ-WRITE The mailbox is open read-write, or its access while
open has changed from read-only to read-write.
TRYCREATE An APPEND or COPY attempt failed because the target
mailbox does not exist. The server sends this as a
hint to the client that the operation would probably
succeed if the mailbox is first created by means of
the CREATE command.
UNSEEN Followed by a decimal number, indicates the number
of the first unread message. This is intended to be
used with certain mailbox formats to assist the user
in finding the first unread message in those cases
where "unread" and "recent" are separate concepts.
hex string A hexadecimal string is returned as a special
information token to represent a Kerberos return
authenticator. This only occurs in response to a
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LOGIN command that uses Kerberos authentication.
Other special information tokens defined by particular client or
server implementations should be prefixed with an "X" until they are
added to a revision of this protocol.
tag OK resp_text
This response indicates successful completion of the command with
that tag. The response text may be useful in a protocol telemetry
log for debugging purposes.
EXAMPLES: C: a001 LOGIN SMITH SESAME
S: a001 OK LOGIN completed
C: a002 EXAMINE old_archive
...
S: a002 OK [READ-ONLY] EXAMINE completed
tag NO resp_text
This response indicates unsuccessful completion of the command
with that tag. The response text is a line of human-readable text
that probably should be displayed to the user in an error report
by the client.
EXAMPLE: C: a002 SELECT garply
S: a002 NO SELECT failed: no such mailbox as "garply"
tag BAD resp_text
This response indicates faulty protocol received from the client.
The response text is a line of human-readable text that should be
recorded in any telemetry as part of a bug report to the
maintainer of the client.
EXAMPLES: C: quit
S: quit BAD Missing command
C: a002 BLURDYBLOOP
S: a002 BAD Command unrecognized: BLURDYBLOOP
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Status Responses
This group of responses convey status information, and are always
untagged. The response text is a line of human readable text. As
with command completion responses, the human readable text may have
an optional prefix inside square brackets that conveys special
information between cooperating servers and clients.
* PREAUTH resp_text
This response is one of three possible greetings at session
startup. It indicates that the session has already been
authenticated by external means and thus no LOGIN command is
needed.
* OK resp_text
This response indicates an information message from the server.
It does not indicate completion of any particular request, nor is
it necessarily related to any request. The text is a line of
human-readable text that should be presented to the user as an
information message.
This response is also one of three possible greetings at session
startup. It indicates that the session is not yet authenticated
and that a LOGIN command is needed.
EXAMPLE: C: A002 SELECT INBOX
S: ...
S: * OK [UNSEEN 17] Message 17 is the first unseen message
S: A002 OK SELECT completed
* NO resp_text
This response indicates a warning message from the server. It
does not indicate completion of any request, nor is it necessarily
related to any request. The text is a line of human-readable text
that should be presented to the user as a warning of improper
operation.
EXAMPLES: S: * NO Disk is full, please delete unnecessary data
S: * NO Server problem #23, please contact your administrator
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* BAD resp_text
This response indicates a serious error message from the server.
It does not indicate completion of any request, nor is it
necessarily related to any request. It may also indicate a faulty
command from the client in which a tag could not be parsed. The
text is a line of human-readable text that should be presented to
the user as a serious or possibly fatal error.
EXAMPLE: S: * BAD Command line too long
* BYE text
This response indicates that the server is about to close the
connection. The text is a line of human-readable text that should
be displayed to the user in a status report by the client. This
may be sent as part of a normal logout sequence, or as a panic
shutdown announcement by the server. It is also used by some
servers as an announcement of an inactivity autologout.
This response is also one of three possible greetings at session
startup. It indicates that the server is not willing to accept a
session from this client.
EXAMPLE: S: * BYE Autologout; idle for too long
Data responses
This group of responses convey data about the mailbox or messages
inside the mailbox and are always untagged. This is how message data
are transmitted from the server to the client, often as a result of a
command with the same name.
* LIST mstring
This response occurs as a result of a LIST command. The mstring
contains information about a name which matches the pattern in the
command.
EXAMPLE: C: A002 LIST "~/Mail/" "%"
S: * LIST (\Noselect) "/" ~/Mail/foo
S: * LIST () "/" ~/Mail/meetings
S: A002 OK LIST completed
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* LSUB mstring
This response occurs as a result of a LSUB command. The mstring
contains information about a name which matches the pattern in the
command.
EXAMPLE: C: A002 LSUB "#news." "comp.mail.*"
S: * LSUB () "." #news.comp.mail.mime
S: * LSUB () "." #news.comp.mail.misc
S: A002 OK LSUB completed
* SEARCH number(s)
This response occurs as a result of a SEARCH command. The
number(s) refer to those messages that match the search criteria.
Each number is delimited by a space.
EXAMPLE: C: A003 SEARCH from "Jones"
S: * SEARCH 2 3 6
S: A003 SEARCH completed
* FLAGS flag_list
This response occurs as a result of a selection command. The flag
list are the list of flags (at a minimum, the system-defined
flags) that are applicable for this mailbox. Flags other than the
system flags are a function of the server implementation.
EXAMPLE: C: A002 SELECT INBOX
S: * FLAGS (\Answered \Flagged \Deleted \Seen \Junked)
...
* number message_data
This response may occurs as a result of any command while a
mailbox is selected. The message_data is one of the following:
EXISTS The number of messages in the mailbox.
RECENT The number of messages that have arrived since the
previous time this mailbox was read.
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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.
An unsolicited EXPUNGE response MUST NOT be sent except
while responding to a request other than FETCH, STORE,
or SEARCH. All references to message numbers sent after
an unsolicited EXPUNGE response are adjusted to reflect
the effect of the expunge.
Discussion: a potential ambiguity exists with
the FETCH, STORE, and SEARCH requests if the
server is permitted to send unsolicited EXPUNGE
responses. This is because these requests can be
streamed. If two successive FETCH requests are
streamed, and if during the time of the processing
of the first request there is an expunge response,
then the sequence of the second request is no
longer valid.
FETCH data
This is the principal means that data about a message
are returned to the client. The data are in an
S-expression form, and consists of a sequence of pairs
of data item name and their values. The current data
items are:
BODY Similar to BODYSTRUCTURE, but without the extension
data.
BODY[section] A string expressing the body contents of the
specified section. The string should be
interpreted by the client according to the
content transfer encoding, body type, and subtype.
Note that non-textual data are transfer encoded;
therefore, the string is likely to be 7-bit
US-ASCII. This is NOT necessarily the byte size
or character set of the interpreted result.
BODYSTRUCTURE An S-expression format list that describes the body
structure of a message. This is computed by the
server by parsing the RFC 822 header and body into
the component parts, defaulting various fields
as necessary.
Multiple parts are indicated by S-expression
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nesting. Instead of a body type as the first element
of the list there is a nested body. The second
element of the list is the multipart subtype (mixed,
digest, parallel, alternative, etc.).
Extension data follows the multipart subtype.
Extension data is never returned with the older
BODY fetch, but may be returned with a BODYSTRUCTURE
fetch. Extension data, if present, must be in the
defined order.
The extension data of a multipart body part are in
the following order:
body parameter list - an S-expression list of
attribute/value pairs [e.g. (foo bar baz rag)
where "bar" is the value of "foo" and "rag" is
the value of "baz"] as defined in MIME.
Any subsequent extension data are not yet defined
in this version of the protocol. Such extension data
consist of zero or more NILs, strings, numbers,
and/or potentially nested lists of such data. Clients
which do a BODYSTRUCTURE fetch MUST be prepared to
accept such extension data. Servers MUST NOT send
such extension data until it has been defined by a
revision of this protocol.
The basic fields of a non-multipart body part are in
the following order:
body type - a string giving the content type name
as defined in MIME
body subtype - a string giving the content subtype
name as defined in MIME
body parameter list - an S-expression list of
attribute/value pairs [e.g. (foo bar baz rag)
where "bar" is the value of "foo" and "rag" is
the value of "baz"] as defined in MIME.
body id - a string giving the content id as
defined in MIME.
body description - a string giving the content
description as defined in MIME.
body encoding - a string giving the content
transfer encoding as defined in MIME.
body size - a number giving the size of the
body in octets. Note that this size is the
size in its transfer encoding and not the
resulting size after any decoding.
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A body type of type MESSAGE and subtype RFC822
contains, immediately after the basic fields,
the envelope structure, body structure, and
size in text lines of the encapsulated message.
A body type of type TEXT contains, immediately
after the basic fields, the size of the body
in text lines. Note that this size is the size
in its transfer encoding and not the resulting
size after any decoding.
Extension data follows the basic fields and the
type-specific fields listed above. Extension data
is never returned with the older BODY fetch, but
may be returned with a BODYSTRUCTURE fetch.
Extension data, if present, must be in the defined
order.
The extension data of a non-multipart body part are
in the following order:
body MD5 - a string giving the content MD5 value
as defined in MIME
Any subsequent extension data are not yet defined
in this version of the protocol, and would be in the
form described above under multipart extension data.
ENVELOPE An S-expression format list that describes the
envelope structure of a message. This 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 structure 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 address structures.
An address structure is an S-expression format list
that describes an electronic mail address. The
fields of an address structure are in the following
order: personal name, source-route (a.k.a. the
at-domain-list in SMTP (RFC 821)), mailbox name, and
host name.
RFC 822 group syntax is indicated by a special
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form of address structure in which the host name
file is NIL. If the mailbox name field is also NIL,
this is an end of group marker (semi-colon in RFC 822
syntax). If the mailbox name field is non-NIL,
this is a start of group marker, and the mailbox
name field holds the group name phrase.
Any field of an envelope or address structure that
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 that are set
for this message. This may include the following
system flags:
\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
\Junked Message has been "junked"
(dismissed as unwanted)
as well as the following special flag:
\Recent Message arrived since the
previous time this mailbox
was read
Discussion: the \Junked flag can be
thought of as the opposite of \Flagged;
that is, junked messages have lower
than normal priority. It differs from
\Deleted in that a junked message is
not removed from the mailbox by EXPUNGE.
One possible of use of the \Junked flag
is with shared mailboxes (e.g. netnews)
where it is impossible or undesirable
to set \Deleted status. For this reason,
\Junked state is always per-user.
INTERNALDATE A string containing the date and time of
final delivery of the message as defined
by SMTP (RFC 821).
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MD5[section] The MD5 checksum of the particular body
section.
RFC822 A string expressing the message in RFC 822
format.
RFC822.HEADER A string expressing the RFC 822 format header
of the message, including the delimiting
blank line between the header and the body.
This is used for the FETCH data items
RFC822.HEADER, RFC822.HEADER.LINES, and
RFC822.HEADER.LINES.NOT (note that a blank
line is always included regardless of header
line restrictions).
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.
UID A number expressing the unique identifier
of the message.
COPY Obsolete. New server implementations MUST NOT transmit
this response. Client implementations SHOULD ignore
this response and not report an error.
STORE data
Obsolete and functionally equivalent to FETCH. New
server implementations MUST NOT transmit this response.
Client implementations SHOULD treat this response as
equivalent to the FETCH response.
Command Continuation Ready Response
+ resp_text
This response indicates that the server is ready to accept the
text of a literal from the client. The text of this response is a
line of human-readable text of the server's choosing (it is
generally never seen by a client's human user).
The purpose of this command is to solve a synchronization problem
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that can occur if a string in a command is a literal. This may
occur when logging in (if the password contains "funny"
characters), and always occurs when using the APPEND command,
since a message consists of multiple lines.
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 for commands that
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 expects it, preserving
client/server synchronization.
No such synchronization protection is provided for literals sent
from the server to the client. Any synchronization problems in
this direction would be caused by a bug in the client or server.
EXAMPLE: C: A001 LOGIN SMITH {7}
S: + Ready for additional command text
C: fat man
S: A001 OK LOGIN completed
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Sample IMAP4 session
The following is a transcript of an IMAP4 session. A long line in
this sample is broken for editorial clarity.
S: * OK IMAP4 Service Ready
C: a001 login mrc secret
S: a001 OK LOGIN completed
C: a002 select inbox
S: * 18 EXISTS
S: * FLAGS (\Answered \Flagged \Deleted \Seen \Junked)
S: * 2 RECENT
S: * OK [UNSEEN 17] Message 17 is the first unseen message
S: a002 OK [READ-WRITE] SELECT completed
S: a003 fetch 12 full
S: * 12 FETCH (FLAGS (\Seen) INTERNALDATE "14-Jul-1993 02:44:25 -0700"
RFC822.SIZE 4282 ENVELOPE ("Wed, 14 Jul 1993 02:23:25 -0700 (PDT)"
"IMAP4 WG mtg summary and minutes"
(("Terry Gray" NIL "gray" "cac.washington.edu"))
(("Terry Gray" NIL "gray" "cac.washington.edu"))
(("Terry Gray" NIL "gray" "cac.washington.edu"))
((NIL NIL "imap" "cac.washington.edu"))
((NIL NIL "minutes" "CNRI.Reston.VA.US")
("John Klensin" NIL "KLENSIN" "INFOODS.MIT.EDU")) NIL NIL
"<B27397-0100000@cac.washington.edu>")
BODY ("TEXT" "PLAIN" ("CHARSET" "US-ASCII") NIL NIL "7BIT" 3028 92))
S: a003 OK FETCH completed
C: a004 fetch 12 rfc822.header
S: * 12 FETCH (RFC822.HEADER {346}
S: Date: Wed, 14 Jul 1993 02:23:25 -0700 (PDT)
S: From: Terry Gray <gray@cac.washington.edu>
S: Subject: IMAP4 WG mtg summary and minutes
S: To: imap@cac.washington.edu
S: cc: minutes@CNRI.Reston.VA.US, John Klensin <KLENSIN@INFOODS.MIT.EDU>
S: Message-Id: <B27397-0100000@cac.washington.edu>
S: Mime-Version: 1.0
S: Content-Type: TEXT/PLAIN; CHARSET=US-ASCII
S:
S: )
S: a004 OK FETCH completed
C: a005 store 12 +flags \deleted
S: * 12 FETCH (FLAGS (\Seen \Deleted))
S: a005 OK +FLAGS completed
C: a006 logout
S: * BYE IMAP4 server terminating connection
S: a006 OK LOGOUT completed
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Design Discussion
IMAP4 is a textual protocol. The use of MIME encoding in IMAP4 makes
it possible to support 8-bit textual and binary mail.
IMAP4 implementations MAY transmit 8-bit or multi-octet characters in
literals, but should do so only when the character set is identified.
For example, 8-bit characters are specifically permitted in MIME body
parts (fetching BODY[section]) of type TEXT. 8-bit characters are
also permitted in the argument to APPEND.
Servers MUST NOT transmit 8-bit characters in RFC822.HEADER fetch
results. Servers MUST NOT transmit 8-bit characters in RFC822.TEXT
(and by extension RFC822) fetch results, unless there are MIME data
in the message that identify the character sets of all 8-bit data.
Because 8-bit characters are permitted in the argument to APPEND, a
server that is unable to preserve 8-bit data properly MUST be able to
reversibly convert 8-bit APPEND data to 7-bit using MIME.
Although a BINARY body encoding is defined, IMAP4 does not permit
unencoded binary strings. A "binary string" is any string with NUL
characters; a string with an excessive amount of CTL characters may
also be considered to be binary. The mixing of unencoded binary in
the same stream as textual commands would make the protocol more
vulnerable to synchronization problems. Implementations MUST encode
binary data into BASE64 before transmitting it with IMAP4.
When operating in the online model, an IMAP4 client should maintain a
local cache of data from the mailbox. This cache is an incomplete
model of the mailbox, and at startup is generally empty. As the
client processes all untagged data, it updates the cache based on
this data. When a tagged response arrives, the client's cache has
been updated from the associated request.
Note that a server can send data that the client did not request,
such as mailbox size or flag updates. A server MUST send mailbox
size updates automatically while processing a command. A server
SHOULD send message flag updates automatically, without requiring the
client to request such updates explicitly.
Regardless of what implementation decisions a client may take on
caching, a client MUST record EXISTS and RECENT updates and MUST NOT
assume that a CHECK or NOOP command will return EXISTS or RECENT
information.
Although it is permitted for a server to send an unsolicited response
while there is no command in progress, this practice SHOULD NOT be
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followed because of flow control considerations. It can cause an
incautious implementation to deadlock. A deadlock is avoided if
either of the following conditions are true: (1) except for the
greeting, the server never sends responses while there is no command
in progress; (2) the server process is capable of reading commands
while sending data. The latter condition generally requires either a
multi-threading server implementation or use of a polling facility
and non-blocking I/O.
If a server has an inactivity autologout timer, that timer MUST be of
at least 30 minutes' duration. The receipt of a NOOP command from
the client during that interval should suffice to reset the
autologout timer. Periodic transmission of a NOOP from the client
during periods of inactivity also has the benefit of avoiding the
possible deadlock noted above.
It is frequently asked why there is no message posting function in
IMAP4. Message posting is orthogonal to the scope of a mail access
protocol and detracts from its primary focus. SMTP (RFC 821)
provides the minimal functionality needed for message posting without
losing valuable capabilities (such as blind carbon copies). Any
message posting function in IMAP4 would need, at a minimum, to
provide equivalent functionality.
At the time of the writing of this document, an extensive set of
extensions to SMTP (RFC 821) is in the Internet standards process.
Should those extensions become an Internet Standard it would be
necessary to revise IMAP4 again to provide corresponding
capabilities, were a message posting facility to be included in
IMAP4. In other words, a duplication of effort would be required
each time a change is made to message transport technology.
Another undesirable aspect of message posting in IMAP4 occurs when a
remote server is used. It is unlikely that a client would support
multiple means of posting a message. It adds excessive size and
complexity that can not be afforded, particularly on smaller
machines. It also can lead to poor performance. Consider a client
connecting to an IMAP4 server over an interactive satellite link to a
foreign country. A local message posting (SMTP (RFC 821)) server is
available that uses a lower-cost batched link. Here, it would be
wasteful to use the interactive link for posting.
Message posting to IMAP4 has been suggested as a means of
authenticating postings. The problem is that access authentication
credentials are not necessarily the same as posting authentication
credentials. At some sites, the disclosure of a portion of access
authentication credentials in a mail message (as a "From" or "Sender"
address) may be a serious security breach of greater significance
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than forged mail.
The Internet message transport infrastructure has no concept of
authentication credentials, and neither authentication syntax nor
semantics are transferred within a message. As a result, any attempt
at authenticating a message via posting authentication is completely
ineffective once the message leaves the authenticating server; any
indication of authentication in the message can easily be reproduced
further down the line. Public-key based message authentication
systems such as Privacy Enhanced Mail are now under development to
address this problem.
IMAP4 does not address problems with multiple IMAP4 servers at a
single site, access control lists, and mobility of client
configuration and address book information. These and other issues
are being considered for a companion protocol.
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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 (SPACE) and
not a comma.
Except as noted otherwise, all alphabetic characters in the IMAP4
protocol are case-insensitive. For example, "LOGIN", "login" and
"lOgIn" all refer to the same command, and \FLAGGED, \Flagged, and
\FlAgGeD all refer to the same flag. The use of upper or lower case
characters to define token strings is for editorial clarity only,
although they may be construed as defining a suggested usage.
Implementations MUST accept these strings in a case-insensitive
fashion.
Syntax marked as obsolete may be encountered with implementations
written for an older version of this specification. New
implementations SHOULD accept obsolete syntax as input, but MUST NOT
otherwise use it.
address ::= "(" addr_name SPACE addr_adl SPACE addr_mailbox
SPACE addr_host ")"
addr_adl ::= nstring
addr_host ::= nstring
;; NIL indicates RFC 822 group syntax
addr_mailbox ::= nstring
;; NIL indicates end of RFC 822 group; if non-NIL
;; and addr_host is NIL, holds RFC 822 group name
addr_name ::= nstring
append ::= "APPEND" SPACE mailbox [SPACE flag_list]
[SPACE date_time] SPACE literal
astring ::= atom / string
atom ::= 1*<any CHAR except specials, SPACE, and CTLs>
body ::= "(" body_structure ")"
body2 ::= "(" body2_structure ")"
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body2_extension ::= nstring / number / "(" 1#body2_extension ")"
;; Future expansion. Clients MUST accept body2
;; extension fields. Servers MUST NOT generate
;; body2 extension fields.
body2_md5 ::= nstring
;; reserved for MD5 checksum
body2_multipart ::= 1*body2 SPACE body_subtype SPACE body_parameter
[SPACE 1#body2_extension]
body2_structure ::= body2_terminal / body2_multipart
body2_terminal ::= body_terminal SPACE body2_md5 [SPACE 1#body2_extension]
body_basic ::= body_type_basic SPACE body_subtype SPACE body_fields
body_fields ::= body_parameter SPACE body_id SPACE body_description
SPACE body_encoding SPACE body_size
body_description
::= nstring
body_encoding ::= <"> ("7BIT" / "8BIT" / "BINARY" / "BASE64"/
"QUOTED-PRINTABLE") <"> / string
body_id ::= nstring
body_msg ::= body_msg_822 / body_msg_other
body_msg_822 ::= body_type_msg SPACE body_subtyp_822 SPACE body_fields
SPACE envelope SPACE body SPACE body_size_lines
body_msg_other ::= body_type_msg SPACE body_subtype SPACE body_fields
;; subtype MUST NOT be "RFC822"
body_multipart ::= 1*body SPACE body_subtype
body_parameter ::= nil / "(" 1#(string string) ")"
body_section ::= number / number "." body_section
body_size ::= number
;; size in octets
body_size_lines ::= number
body_structure ::= body_terminal / body_multipart
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body_subtype ::= string
body_subtyp_822 ::= <"> "RFC822" <">
body_terminal ::= body_basic / body_msg / body_text
body_text ::= body_type_text SPACE body_subtype SPACE body_fields
SPACE body_size_lines
body_type_basic ::= <"> ("APPLICATION" / "AUDIO" / "IMAGE" / "VIDEO") <"> /
string
body_type_msg ::= <"> "MESSAGE" <">
body_type_text ::= <"> "TEXT" <">
CHAR ::= <any 7-bit US-ASCII character except NUL, 0x01 - 0x7f>
CHAR8 ::= <any 8-bit octet except NUL, 0x01 - 0xff>
check ::= "CHECK"
copy ::= "COPY" SPACE sequence SPACE mailbox
CR ::= <ASCII CR, carriage return, 0x0C>
create ::= "CREATE" SPACE mailbox
;; use of INBOX gives a NO error
CRLF ::= CR LF
CTL ::= <any ASCII control character and DEL, 0x00 - 0x1f, 0x7f>
date ::= date_text / <"> date_text <">
date_day ::= 1*2DIGIT
;; day of month
date_day_fixed ::= (SPACE 1DIGIT) / 2DIGIT
;; fixed-format version of date_day
date_month ::= "Jan" / "Feb" / "Mar" / "Apr" / "May" / "Jun" /
"Jul" / "Aug" / "Sep" / "Oct" / "Nov" / "Dec"
date_text ::= date_day "-" date_month "-" (date_year / date_year_old)
date_year ::= 4DIGIT
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date_year_old ::= 2DIGIT
;; Obsolete, (year - 1900)
date_time ::= <"> (date_time_new / date_time_old) <">
date_time_new ::= date_day_fixed "-" date_month "-" date_year SPACE
time SPACE zone
date_time_old ::= date_day_fixed "-" date_month "-" date_year_old SPACE
time "-" zone_old
;; Obsolete
delete ::= "DELETE" SPACE mailbox_other
DIGIT ::= <any ASCII decimal digit, 0x30 - 0x39>
DIGIT_HEX :: DIGIT / "a" / "b" / "c" / "d" / "e" / "f"
envelope ::= "(" env_date SPACE env_subject SPACE env_from SPACE
env_sender SPACE env_reply-to SPACE env_to SPACE
env_cc SPACE env_bcc SPACE env_in-reply-to SPACE
env_message-id ")"
env_bcc ::= nil / "(" 1*address ")"
env_cc ::= nil / "(" 1*address ")"
env_date ::= nstring
env_from ::= nil / "(" 1*address ")"
env_in-reply-to ::= nstring
env_message-id ::= nstring
env_reply-to ::= nil / "(" 1*address ")"
env_sender ::= nil / "(" 1*address ")"
env_subject ::= nstring
env_to ::= nil / "(" 1*address ")"
examine ::= "EXAMINE" SPACE mailbox
expunge ::= "EXPUNGE"
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fetch ::= "FETCH" SPACE sequence SPACE ("ALL" / "FULL" /
"FAST" / fetch_att / "(" 1#fetch_att ")")
fetch_att ::= fetch_att_lines / fetch_att_other / fetch_att_text
fetch_att_lines ::= "RFC822.HEADER.LINES" SPACE header_line_list /
"RFC822.HEADER.LINES.NOT" SPACE header_line_list /
fetch_att_other ::= "BODY" / "BODYSTRUCTURE" / "ENVELOPE" / "FLAGS" /
"INTERNALDATE" / "MD5[" body_section "]" /
"RFC822.SIZE" / "UID"
fetch_att_text ::= "BODY[" body_section "]" /
"BODY.PEEK[" body_section "]" / "RFC822" /
"RFC822.PEEK" / "RFC822.HEADER" / "RFC822.TEXT" /
"RFC822.TEXT.PEEK"
flag_autocreate ::= "\*"
flag_fetchable ::= "(" 0#(flag_storable / flag_recent) ")"
flag_list ::= "(" flag_storable ")"
flag_permanent ::= "(" 0#(flag_storable / flag_autocreate) ")"
flag_recent ::= "\Recent"
flag_storable ::= 1#(flag_system / flag_user)
flag_system ::= "\Answered" / "\Flagged" / "\Deleted" / "\Seen" /
"\Junked" / "\" atom
flag_user ::= atom
greeting ::= "*" SPACE (resp_cond_auth / resp_cond_bye) CRLF
header_line ::= astring
header_line_list
::= "(" 1#header_line ")"
inbox ::= "INBOX"
;; case-independent, but SHOULD be upper-case
kerberos_authenticator
::= 1*DIGIT_HEX
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kerberos_response
::= 1*DIGIT_HEX
LF ::= <ASCII LF, line feed, 0x0A>
list ::= "LIST" SPACE list_reference SPACE list_mailbox
list_mailbox ::= mailbox
;; may include wspecials
list_reference ::= mailbox_other
literal ::= "{" number "}" CRLF 1*CHAR8
;; The number represents the number of CHAR8 octets.
login ::= "LOGIN" SPACE userid SPACE password
logout ::= "LOGOUT"
lsub ::= "LSUB" SPACE list_reference SPACE list_mailbox
mailbox ::= inbox / mailbox_other
mailbox_other ::= astring
;; May not be INBOX (in any case). Should not
;; include wspecials. May be case-dependent
;; as a function of server implementation
mbx_level_delim ::= text_char
;; hierarchy level delimiter returned by LIST
message_data ::= number SPACE (msg_exists / msg_recent / msg_expunge /
msg_fetch / msg_obsolete)
msg_copy ::= "COPY"
;; Obsolete
msg_exists ::= "EXISTS"
msg_expunge ::= "EXPUNGE"
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msg_fetch ::= "FETCH" SPACE "(" 1#("BODY" SPACE body /
"BODYSTRUCTURE" SPACE body2 /
"BODY[" body_section "]" nstring /
"ENVELOPE" SPACE envelope /
"FLAGS" SPACE flag_fetchable /
"INTERNALDATE" SPACE date_time /
"MD5[" body_section "]" SPACE nstring /
"RFC822" SPACE nstring /
"RFC822.HEADER" SPACE nstring /
"RFC822.SIZE" SPACE number /
"RFC822.TEXT" SPACE nstring /
"UID" SPACE uniqueid) ")"
msg_obsolete ::= msg_copy / msg_store
;; Obsolete untagged data responses
msg_recent ::= "RECENT"
msg_store ::= "STORE" SPACE "(" 1#("FLAGS" SPACE flag_fetchable ")"
;; Obsolete
mstring ::= mstring_attr SPACE mstring_sep SPACE mailbox
;; Represents a mailbox
mstring_attr ::= "(" 0#("\Noinferiors" / "\Noselect" / "\" atom) ")"
mstring_sep ::= <"> quoted_char <"> / nil
nil ::= "NIL"
nonmessage_data ::= "LIST" SPACE mstring / "LSUB" SPACE mstring /
"SEARCH" [SPACE 1#number] / "FLAGS" SPACE flag_list
noop ::= "NOOP"
nstring ::= nil / string
number ::= 1*DIGIT
partial ::= "PARTIAL" SPACE number SPACE fetch_att_text SPACE
number SPACE number
password ::= astring / "@KERBEROS:" kerberos_authenticator
QCHAR ::= <any CHAR except qspecials, CR, and LF>
qspecials ::= <"> / "\"
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quoted_char ::= QCHAR / "\" qspecials
quoted_string ::= <"> *quoted_char <">
ready ::= "+" SPACE resp_text
rename ::= "RENAME" SPACE mailbox SPACE mailbox_other
request ::= tag SPACE (request_auth / request_authed / request_open)
;; modal based on state
request_any ::= noop / logout
;; valid in all modes
request_auth ::= request_any / login
;; valid only when in not authenticated mode
request_authed ::= request_any / create / delete / rename / list / lsub /
subscribe / unsubscribe / select / examine / append /
x_command
;; valid only when in authenticated or mailbox open mode
request_open ::= request_authed / check / expunge / copy / fetch /
partial / store / uid / search / x_command
;; valid only when in mailbox open mode
response ::= *<response_data> response_done
response_data ::= "*" SPACE (resp_cond_state / resp_cond_bye /
message_data / nonmessage_data) CRLF
response_done ::= response_tagged / response_fatal
response_fatal ::= "*" SPACE resp_cond_bye CRLF
response_tagged ::= tag SPACE resp_cond_state CRLF
resp_cond_auth ::= ("OK" / "PREAUTH") SPACE resp_text
;; authentication condition
resp_cond_bye ::= "BYE" SPACE resp_text
;; server will disconnect condition
resp_cond_state ::= ("OK" / "NO" / "BAD") SPACE resp_text
;; status condition
resp_text ::= [resp_token SPACE] text_line
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resp_token ::= "[" resp_token_type "]"
resp_token_type ::= "PARSE" / "PERMANENTFLAGS" SPACE flag_permanent /
"READ-ONLY" / "READ-WRITE" / "TRYCREATE" /
"UNSEEN" SPACE number/ "X" atom / kerberos_response
search ::= search_new / search_old
search_charset ::= "CHARSET" SPACE astring
;; Must be registered with IANA as a MIME character set.
search_criterion
::= search_crit_new / search_crit_old /
"(" search_program ")"
search_crit_new ::= sequence / "UID" SPACE sequence /
"HEADER" SPACE header_line SPACE astring / "JUNKED" /
"LARGER" SPACE number / "SENTBEFORE" SPACE date /
"SENTON" SPACE date / "SENTSINCE" SPACE date /
"SMALLER" SPACE number / "UNJUNKED"
search_crit_old ::= "ALL" / "ANSWERED" / "BCC" SPACE astring /
"BEFORE" SPACE date / "BODY" SPACE astring /
"CC" SPACE astring / "DELETED" / "FLAGGED" /
"FROM" space astring / "KEYWORD" SPACE flag_user /
"NEW" / "OLD" / "ON" SPACE date / "RECENT" / "SEEN" /
"SINCE" SPACE date / "SUBJECT" SPACE astring /
"TEXT" SPACE astring / "TO" SPACE astring /
"UNANSWERED" / "UNDELETED" / "UNFLAGGED" /
"UNKEYWORD" SPACE flag_user / "UNSEEN"
search_new ::= "SEARCH" SPACE [search_charset SPACE] search_program
search_old ::= "SEARCH" SPACE 1#search_crit_old
search_program ::= 1#(search_criterion / "NOT" SPACE search_criterion /
"OR" SPACE search_criterion SPACE search_criterion)
select ::= "SELECT" SPACE mailbox
sequence ::= sequence_num / (sequence "," sequence) /
(sequence_num ":" sequence_num)
;; identifies a set of messages by consecutive numbers
;; from 1 to the number of messages in the mailbox.
;; Comma delimits individual numbers, colon delimits
;; between two numbers inclusive.
;; Example: 2,4:7,9,12:* is 2,4,5,6,7,9,12,13,14,15 for
;; a mailbox with 15 messages.
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sequence_num ::= number / "*"
;; * is the largest number in use. For message numbers,
;; it is the number of messages in the mailbox. For
;; unique identifiers, it is the unique identifier of the
;; message whose message number is the number of messages
;; in the mailbox (that is, the last message).
SPACE ::= <ASCII SP, space, 0x20>
specials ::= "(" / ")" / "{" / qspecials
store ::= "STORE" SPACE sequence SPACE store_att SPACE store_flags
store_att ::= "+FLAGS" / "-FLAGS" / "FLAGS"
store_flags ::= flag_list / flag_storable
string ::= quoted_string / literal
subscribe ::= "SUBSCRIBE" SPACE mailbox
tag ::= 1*<any CHAR except "*", "+", specials, SPACE, and CTLs>
text_char ::= <any CHAR except CR and LF>
text_line ::= 1*text_char
time ::= 2DIGIT ":" 2DIGIT ":" 2DIGIT
;; hours minutes seconds
uid ::= "UID" SPACE (copy / fetch / search / store)
;; uniqueids used instead of message numbers
uniqueid ::= number
;; strictly ascending
unsubscribe ::= "UNSUBSCRIBE" SPACE mailbox
userid ::= astring
wspecials ::= "%" / "?" / "*"
x_command ::= "X" atom <optional arguments>
;; experimental expansion commands
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zone ::= ("+" / "-") 4DIGIT
;; Signed four-digit value of hhmm representing
;; hours and minutes west of Greenwich (that is,
;; (the amount that the given time differs from
;; Universal Time). Subtracting the timezone
;; from the given time will give the UT form.
;; The Universal Time zone is "+0000".
zone_old ::= "UT" / "GMT" / "Z" / ;; +0000
"AST" / "EST" / "CST" / "MST" / ;; -0400 to -0700
"PST" / "YST" / "HST" / "BST" / ;; -0800 to -1100
"ADT" / "EDT" / "CDT" / "MDT" / ;; -0300 to -0600
"PDT" / "YDT" / "HDT" / "BDT" / ;; -0700 to -1000
"A" / "B" / "C" / "D" / "E" / "F" / ;; +0100 to +0600
"G" / "H" / "I" / "K" / "L" / "M" / ;; +0700 to +1200
"N" / "O" / "P" / "Q" / "R" / "S" / ;; -0100 to -0600
"T" / "U" / "V" / "W" / "X" / "Y" ;; -0700 to -1200
;; Obsolete
A protocol session is as follows:
Server: greeting
*<Client: request (first part, if it contains a literal)
*<Server: ready
Client: request (next part)
>
Server: response
>
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Compatibility Notes
This is a summary of hints and recommendations to enable an IMAP4
implementation, written to this specification, to interoperate with
implementations that conform to earlier specifications. None of
these hints and recommendations are required by this specification;
implementors must decide for themselves whether they want their
implementation to fail if it encounters old software.
IMAP4 has been designed to be upwards compatible with earlier
specifications. For the most part, IMAP4 facilities that were not in
earlier specifications should be invisible to clients unless the
client asks for the facility.
In some cases, older servers may support some of the capabilities
listed as being "new in IMAP4" as experimental extensions to the
IMAP2 protocol described in RFC 1176.
This information may not be complete; it reflects current knowledge
of server and client implementations as well as "folklore" acquired
in the evolution of the protocol.
IMAP4 client interoperability with old servers
In general, a client should be able to discover whether a server
supports a facility by trial-and-error; if an attempt to use a
facility generates a BAD response, the client can assume that the
server does not support the facility.
Some servers may disable certain commands as a matter of
intentional site policy. Such servers should return a NO response
to disabled commands instead of a BAD response.
A quick way to check whether a server implementation supports this
specification is to try a UID FETCH 0 UID command. An OK or NO
response would indicate a server that conforms to this
specification; a BAD response would indicate an older server.
Known problems are noted below:
The CREATE, DELETE, and RENAME commands are new in IMAP4, and
may not be present in old servers. A safe mechanism to test
whether these commands are present is to try a CREATE INBOX
command. If the response is NO, these commands are supported
by the server. If the response is BAD, they are not. If the
response is OK, the server's implementation is broken, since
creating INBOX is not permitted.
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The FIND and BBOARD commands of RFC 1176 are removed from
IMAP4. The FIND command has been replaced with the new LSUB
and LIST commands. There is no equivalent to the BBOARD
command, which provided a separate namespace with implicit
restrictions on what may be done in that namespace.
The LSUB and LIST commands are new in IMAP4. A BAD response to
these commands indicates a server that does not support mailbox
listing. Note that the definition of the LSUB and LIST
commands differ significantly from the FIND command of RFC
1176.
The SUBSCRIBE and UNSUBSCRIBE commands are new in IMAP4. A
server that supports LSUB will also support the SUBSCRIBE and
UNSUBSCRIBE commands. The experimental IMAP2bis version had
SUBSCRIBE and UNSUBSCRIBE commands with a different syntax, but
did not support the LSUB command.
The EXAMINE command is new in IMAP4. A BAD response to this
command indicates a server that does not support an explicit
read-only mode of access, and a SELECT command should be used
instead.
Older server implementations may automatically create the
destination mailbox on COPY if that mailbox does not already
exist. This was how a new mailbox was created in older
specifications. If the server does not support the CREATE
command (see above for how to test for this), it will probably
create a mailbox on COPY.
The APPEND command is new in IMAP4. A way to see if this
command is implemented is to try to append a zero-length stream
to a mailbox name that is known not to exist (or at least,
highly unlikely to exist) on the remote system.
Although IMAP4 clients SHOULD avoid asking for the same data
more than once (by having a client-based cache of data returned
by the server), this is not a requirement of the protocol.
However, IMAP4 clients MUST cache data from the EXISTS and
RECENT unsolicited responses. Only the SELECT command is
guaranteed to return EXISTS/RECENT information.
The use of * in a sequence to indicate the largest number in
use is new in IMAP4. A BAD response to an attempt to fetch
data using * as a sequence number indicates a server that does
not support this capability. With message number sequences,
the same effect can be obtained by using the number of messages
in the mailbox from the EXISTS unsolicited response.
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The BODY, BODY[section], MD5[section], and FULL fetch data
items are new in IMAP4. A BAD response to an attempt to fetch
these data items indicates a server that does not support
server-based MIME parsing. Servers supporting the experimental
IMAP2bis version may lack MD5[section] but have the other
capabilities.
The BODYSTRUCTURE fetch data item is new in IMAP4. A BAD
response to an attempt to fetch this data item indicates a
server that does not support extensible results from server-
based MIME parsing. Servers running the experimental IMAP2bis
version may support the older, non-extensible, BODY fetch data
item. A client should attempt this data item before deciding
that the server does not support MIME.
The use of nested part 0 of a part of type MESSAGE in a BODY or
BODYSTRUCTURE fetch to get only the RFC 822 header of the
message is new, and is not in the experimental IMAP2bis
version. A server that returns NIL probably supports IMAP2bis;
with such servers the only way to obtain the RFC 822 header is
to fetch the entire nested message.
The RFC822.HEADER.LINES and RFC822.HEADER.LINES.NOT fetch data
items are new in IMAP4. A BAD response to an attempt to fetch
this data item indicates a server that does not support
selective header fetching. A client should use RFC822.HEADER
and remove the unwanted information.
The BODY.PEEK[section], RFC822.PEEK, and RFC822.TEXT.PEEK fetch
data items are new in IMAP4. A BAD response to an attempt to
fetch one of these data items indicates a server that does not
support fetching of message data without implicitly setting the
\Seen flag. A client should use the corresponding non-PEEK
versions and manually clear the \Seen flag as necessary.
The UID fetch data item and the UID commands are new in IMAP4.
A BAD response to an attempt to use these indicates a server
that does not support unique identifiers.
The PARTIAL command is new in IMAP4. If this command causes a
BAD response, then the client should use the appropriate FETCH
command and ignore the unwanted data.
The SEARCH command has been extended in several ways in IMAP4,
including the addition of a character set specifier for
international searching. A BAD response to a SEARCH command
which attempts to use any search capability other than those
listed in search_old in the grammar indicates a server that
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does not support any of the new SEARCH capabilities. A client
should reformulate its search request using only the searching
options listed in search_old. This may entail doing multiple
searches to achieve the desired results.
IMAP4 client implementations must accept all responses and data
formats documented in this specification, including those
labeled as obsolete. This includes the COPY and STORE
unsolicited responses and the old format of dates and times.
The experimental IMAP2bis version did not provide a way to set
flags or internal date on APPEND. Client implementations which
receive a BAD response to an APPEND command with flag list
and/or internal date should retry the command without any flag
list and internal date arguments.
Older server implementations may not preserve flags or internal
dates on COPY. Some server implementations may not permit the
preservation of certain flags on COPY or their setting with
APPEND as site policy.
The experimental IMAP2bis version specified that the TRYCREATE
special information token is sent as a separate unsolicited OK
response instead of inside the NO response.
IMAP4 server interoperability with old clients
In general, there should be no interoperation problem between a
server conforming to this specification and a well-written client
that conforms to an earlier specification. Known problems are
noted below:
Clients written to use undocumented private server extensions
that are not in any published specification may work poorly
with server implementations that do not have those extensions.
Poor wording in the description of the CHECK command in earlier
specifications implied that a CHECK command is the way to get
the current number of messages in the mailbox. This is
incorrect. A CHECK command does not necessarily result in an
EXISTS response. Clients must remember the most recent EXISTS
value sent from the server, and should not generate unnecessary
CHECK commands.
An incompatibility exists with COPY in IMAP4. COPY in IMAP4
servers does not automatically create the destination mailbox
if that mailbox does not already exist. This may cause
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problems with old clients that expect automatic mailbox
creation in COPY.
The PREAUTH unsolicited response is new in IMAP4. It is highly
unlikely that an old client would ever see this response.
The COPY unsolicited response is obsolete. Clients must not
depend on receiving this response.
The STORE unsolicited response is obsolete. Clients must not
object to receiving a FETCH response instead of this response.
The format of dates and times has changed due to the impending
end of the century. Clients must accept a four-digit year
instead of a two-digit year, and a signed four-digit timezone
value instead of a timezone name. In particular, client
implementations must not treat a date/time as a fixed format
string; nor may they assume that the time begins at a
particular octet.
An incompatibility exists with the use of "\" in quoted
strings. This is best avoided by using literals instead of
quoted strings if "\" or <"> is embedded in the string.
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Acknowledgements
Bill Yeager and Rich Acuff contributed invaluable suggestions in the
evolution of IMAP2 from the original IMAP. James Rice pointed out
several ambiguities in the previous IMAP2 specification.
My colleagues on the Pine team -- Steve Hubert, Laurence Lundblade,
David Miller, and Mike Seibel -- worked long and hard to create a
fantastic email user agent with worldwide popularity. Without their
efforts, IMAP2 would have languished in obscurity. Terry Gray, our
boss, provided much-needed moral support and guidance, while refusing
to let us get away with "good enough" when "great" was possible.
John G. Myers and Chris Newman carefully examined the formal grammar
and identified numerous mistakes and omissions in the drafts of this
specification. Rob Austein contributed important information on the
workings of disconnected access. They also provided invaluable input
towards the overall architecture of the present protocol, and endured
long meetings to reach the present protocol.
The present protocol would not have come into existence without the
assistance of the rest of the IETF IMAP2 working group, in particular
Ned Freed and Adam Treister.
Any mistakes, flaws, or sins of omission in this IMAP4 protocol
specification are, however, strictly my own; and the mention of any
name above does not imply an endorsement.
Security Considerations
Security issues are discussed in this memo only as far as
authentication to access a server are concerned.
Author's Address
Mark R. Crispin
Networks and Distributed Computing, JE-30
University of Washington
Seattle, WA 98195
Phone: (206) 543-5762
EMail: MRC@CAC.Washington.EDU
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