Network Working Group Mark Crispin
Internet Draft: IMAP2bis University of Washington
Obsoletes: RFC 1176, 1064 August 1993
INTERACTIVE MAIL ACCESS PROTOCOL - VERSION 2bis
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
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status of any this or any other Internet Draft.
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
December 1993. Distribution of this draft is unlimited. Comments
are solicited and should be sent to imap@CAC.Washington.EDU.
Introduction
The Interactive Mail Access Protocol, Version 2bis (IMAP2bis) allows
a client to access and manipulate electronic mail on a server.
IMAP2bis is designed to permit manipulations of remote mailboxes as
if they were local. IMAP2bis includes operations for creating,
deleting, and renaming mailbox folders; checking for new mail;
permanently removing messages; setting and clearing flags; RFC 822
and MIME parsing; searching; and selective fetching of message
attributes, texts, and portions thereof.
IMAP2bis does not specify a means of posting mail; this function is
handled by a mail transfer protocol such as SMTP (RFC 821).
IMAP2bis assumes a reliable data stream such as provided by TCP.
When TCP is used, an IMAP2bis 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. IMAP2bis 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 of 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 entire session. No mailbox data
is kept on the client; the client retrieves data from the server as
is needed. IMAP2bis introduces a form of the online model which
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, as
shown in the following table:
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 IMAP2bis was originally designed to accommodate the online
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model, it can support the other two models as well. This makes
possible the creation of clients which 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. due to
travel).
IMAP2bis is designed to transmit message data on demand, and to
provide the facilities necessary for a client to make an intelligent
decision on 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, IMAP2bis provides the capability of server-based
RFC 822 and MIME processing. With this information, it is possible
for a client to determine in advance whether or not it wishes to
retrieve a particular message or part of a message. For example, a
user connected to an IMAP2bis 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 IMAP2bis, 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. With the possible
exception of unique identifiers used in disconnected use operation,
the client initially has no knowledge of mailbox state and learns it
from the IMAP2bis server when a mailbox is selected. This initial
transfer is minimal; the client requests additional state data as it
needs it.
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The Protocol
The IMAP2bis protocol consists of a sequence of client commands and
server responses, with server data interspersed between the
responses. Unlike most Internet protocols, commands and responses
are tagged. That is, a command begins with a unique identifier
(typically a short alphanumeric sequence such as a Lisp "gensym"
function would generate e.g., A0001, A0002, etc.) called a tag. The
response to the command is given the same tag from the server.
Additionally, the server may send an arbitrary amount of "unsolicited
data", which is identified by the special reserved tag of "*". The
unsolicited data mechanism is used to transmit most data in IMAP2bis.
The term "unsolicited data" refers to the fact that the data may have
been transmitted without any explicit request by the client for that
data. No distinction is made in IMAP2bis between data transmitted as
a result of a client command and data which is unilaterally
transmitted by the server. One form of unilaterally transmitted data
which commonly occurs is an alert of a change to the mailbox made by
some process other than the IMAP2bis client or server; for example,
changes in the size of the mailbox (new mail) or in the status of
individual messages.
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 responses, often
interspersed with data.
In general, the command responses do not themselves contain the
requested data, but rather indicate the completion status of the
request. There are three fundamental responses: success (OK), error
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(NO), request faulty or not understood (BAD). The effect of a
command can not be considered complete until a command 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 has
certain other effects; for example, this implies that an EXPUNGE
response can not be transmitted as part of a response to a request
that may be streamed with another response.
If authentication has not yet been completed, it must now be done via
the LOGIN command before any IMAP2bis commands are permitted. The
only other permitted command prior to successful authentication is
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.
Mailbox names are implementation dependent. However, the word
"INBOX" must be implemented to mean the primary or default mailbox
for this user, independent of any other server semantics. It is
permitted for a server not to have an INBOX if there is no concept of
a primary or default mailbox for this user. The name "INBOX" MUST
NOT be used for any other purpose.
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 unsolicited data, followed by an OK
response. The client may terminate access to this mailbox and access
a different one with another selection command.
The client requests mailbox data with FETCH commands, and receives it
via the unsolicited data mechanism. Three major categories of
mailbox data exist.
The first category is data that is 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
placed in the mailbox. The RFC 822 size is the size in bytes 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 which is guaranteed to refer to this message and to none
other and which, unlike IMAP2bis sequence numbers, persists across
sessions.
The second category is data that describes the composition and
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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 IMAP2bis, the RFC 822 header information that may be
fetched is called the "envelope" (not to be confused with SMTP
envelopes). Similarly, the MIME header information that may be
fetched is called the "body". A client can use the parsed envelope
and body and not worry about having to do its own RFC 822 or MIME
parsing.
The third category is textual data, some of which is intended for
direct human viewing. IMAP2bis 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.
In the simplest case, a client can "FETCH RFC822" to get the entire
message without any processing. A more sophisticated 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.
Data structures in IMAP2bis 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 structure).
The client can alter certain data with a STORE command. As an
example, a message is deleted 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 deleted messages,
checking for updated mailbox state, and searching for messages that
match certain criteria. It is also possible to select different
mailboxes, create a new mailbox, and rename or delete an existing
mailbox.
The client should terminate 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
perform 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
IMAP2bis service is made through some link protocol which 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 which
provides authentication through a "trusted host" facility. Another
example would be a manual invocation of an IMAP2bis server from a
logged-in timesharing job.
A pre-authenticated IMAP2bis 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 IMAP2bis Server pre-authenticated as user "Smith"
C: A001 SELECT INBOX
S: * FLAGS (\Answered \Flagged \Deleted \Seen)
S: * 19 EXISTS
S: * 2 RECENT
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 an user identifier and Kerberos authenticator. See the
description of the LOGIN command for more details.
Servers may allow non-authenticated access to certain mailboxes or
bulletin boards. By convention, this is accomplished with a LOGIN
command using the userid "anonymous". A password is still required.
It is implementation-dependent what requirements, if any, are placed
upon 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 in order to be
considered IMAP2bis compliant.
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Definitions of Commands and Responses
Summary of Defined Commands and Responses
Commands || Responses
-------- || -------
tag NOOP || tag OK response_text
tag LOGIN user password || tag NO response_text
tag LOGOUT || tag BAD response_text
tag CREATE mailbox || * PREAUTH response_text
tag DELETE mailbox || * OK response_text
tag RENAME old_mailbox new_mailbox || * NO response_text
tag FIND MAILBOXES pattern || * BAD response_text
tag FIND ALL.MAILBOXES pattern || * BYE response_text
tag FIND BBOARDS pattern || * MAILBOX mstring
tag FIND ALL.BBOARDS pattern || * BBOARD mstring
tag SUBSCRIBE MAILBOX mailbox || * SEARCH 1#number
tag UNSUBSCRIBE MAILBOX mailbox || * FLAGS flag_list
tag SUBSCRIBE BBOARD mailbox || * number EXISTS
tag UNSUBSCRIBE BBOARD mailbox || * number RECENT
tag SELECT mailbox || * number EXPUNGE
tag EXAMINE mailbox || * number FETCH data
tag BBOARD mailbox || * number COPY
tag CHECK || * number STORE data
tag EXPUNGE || + text
tag COPY sequence mailbox ||
tag APPEND mailbox string ||
tag FETCH sequence data ||
tag PARTIAL msgno data start count ||
tag STORE sequence data value ||
tag UID AFTER uniqueid ||
tag UID BEFORE uniqueid ||
tag UID COPY sequence mailbox ||
tag UID FETCH sequence data ||
tag UID STORE sequence data value ||
tag SEARCH search_program ||
tag x_command arguments ||
Note: there is no pairing between commands and responses on the same
line. Any command may result in any number (including none at all)
of any of responses beginning with "*" (referred to as "unsolicited
data"), followed by one tagged response.
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Commands
Certain commands and facilities described below are listed as
"optional". This term is used to identify server capabilities which
may not be present in a minimal server implementation or a server
implementation based upon an older version of this specification.
Such a server is considered to be IMAP2bis compatible (but not
necessarily compliant) if it implements all commands which are not
labeled as "optional".
An IMAP2bis compliant server implementation MUST implement all of the
commands and facilities listed below, whether or not they are
labelled as "optional".
An IMAP2bis compliant client MUST be prepared to accept BAD as a
response to any of the optional commands or facilities, and
substitute alternate or default action in that case.
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.
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 unsolicited 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: a001 LOGIN SMITH SESAME
logs in as user SMITH with password SESAME.
If a server supports authentication via Kerberos, it may accept
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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 IMAP2bis 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.
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.
Mailbox manipulation commands: CREATE, DELETE, RENAME
These commands permit the manipulation of entire mailboxes. Server
support for mailbox manipulation commands is optional.
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.
Creating INBOX is not permitted. If there is a primary or
default mailbox for this user, it MUST exist and be called
INBOX.
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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
prior to permitting deletion; this would prevent the deletion
of a mailbox which 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.
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 which already exists. Any
error in renaming will return a NO response.
Renaming INBOX is permitted. A new, empty INBOX is created in
its place.
FIND commands
The FIND commands return some set of unsolicited MAILBOX or BBOARD
replies, depending upon the type of FIND, that have as their value a
single mailbox name.
Three wildcard characters are defined in the pattern argument. "*"
specifies any number (including zero) characters may match at this
position. "%" and "?" specify a single character may match at this
position. For example, FOO*BAR will match FOOBAR, FOOD.ON.THE.BAR
and FOO.BAR, whereas FOO%BAR and FOO?BAR match only FOO.BAR. "*"
will match all mailboxes.
Server support for all forms of FIND is optional. If a FIND command
returns BAD as a response then the client can make no assumptions
about what mailboxes exist on the server.
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tag FIND MAILBOXES pattern
The FIND MAILBOXES command accepts as an argument a pattern
(including wildcards) that specifies some set of mailbox names
that the user has declared as being "active" or "subscribed."
The exact meaning of this is implementation-dependent, since
the concept of a set of "active" or "subscribed" mailboxes that
is preserved across sessions may not be meaningful for a
particular server or server implementation. If the SUBSCRIBE
MAILBOX and UNSUBSCRIBE MAILBOX commands are implemented then
this command returns the list manipulated by those commands.
EXAMPLE: C: A002 FIND MAILBOXES *
S: * MAILBOX FOOBAR
S: * MAILBOX GENERAL
S: A002 OK FIND completed
tag FIND ALL.MAILBOXES pattern
The FIND ALL.MAILBOXES command is similar to FIND MAILBOXES;
however, it should return a complete list of all mailboxes
available to the user. Data is returned as in FIND MAILBOXES.
The special name INBOX is included in the output from FIND
ALL.MAILBOXES unless INBOX is not supported by this server or
for this user. The criteria for omitting INBOX is whether or
not SELECT INBOX will return failure; it is not relevant
whether or not the user's actual INBOX resides on this or some
other server. FIND MAILBOXES and SUBSCRIBE MAILBOX provide a
mechanism for the user to identify that this is his or her real
INBOX.
If FIND ALL.MAILBOXES is implemented FIND MAILBOXES must also
be implemented, even if it returns the same information. Also,
FIND ALL.MAILBOXES must, at least, return all the mailbox names
that are returned by FIND MAILBOXES.
The exact meaning of this 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.
tag FIND BBOARDS pattern
The FIND BBOARDS command accepts as an argument a pattern that
specifies some set of bulletin board names that the user has
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declared as being "active" or "subscribed." Wildcards are
permitted as in FIND MAILBOXES.
The FIND BBOARDS command will return some set of unsolicited
BBOARD replies that have as their value a single bulletin board
name.
EXAMPLE: C: A002 FIND BBOARDS *
S: * BBOARD FOOBAR
S: * BBOARD GENERAL
S: A002 OK FIND completed
The exact meaning of this is implementation-dependent, since
the concept of a set of "active" or "subscribed" bboards that
is preserved across sessions may not be meaningful for a
particular server or server implementation. If the SUBSCRIBE
BBOARD and UNSUBSCRIBE BBOARD commands are implemented then
this command returns the list manipulated by those commands.
tag FIND ALL.BBOARDS pattern
The FIND ALL.BBOARDS command is similar to FIND BBOARDS;
however, it should return a complete list of all bulletin
boards available to the user. Data is returned as in FIND
BBOARDS.
If FIND ALL.BBOARDS is implemented FIND BBOARDS must also be
implemented, even if it returns the same information. Also,
FIND ALL.BBOARDS must, at least, return all the bboard names
that are returned by FIND BBOARDS.
The exact meaning of this is implementation-dependent, since
the concept of a bounded or deterministic set of "bboards
available to the user" may not be meaningful for a particular
server or server implementation.
Subscription commands
These commands permit the manipulation of mailbox or bulletin board
subscriptions. Server support for subscription commands is optional.
Subscription status should be preserved between sessions.
tag SUBSCRIBE MAILBOX mailbox
The SUBSCRIBE MAILBOX command adds the specified mailbox name
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to the list of "active" or "subscribed" mailboxes as returned
by the FIND MAILBOXES command. This command returns an OK
response only if the subscription is successful.
If SUBSCRIBE MAILBOX is implemented then FIND MAILBOXES must
also be implemented, and FIND ALL.MAILBOXES should be
implemented.
tag UNSUBSCRIBE MAILBOX mailbox
The UNSUBSCRIBE MAILBOX command removes the specified mailbox
name from the list of "active" or "subscribed" mailboxes as
returned by the FIND MAILBOXES command. This command returns
an OK response only if the unsubscription is successful.
If UNSUBSCRIBE MAILBOX is implemented then SUBSCRIBE MAILBOXES
and FIND MAILBOXES must also be implemented, and FIND
ALL.MAILBOXES should be implemented.
tag SUBSCRIBE BBOARD bboard
The SUBSCRIBE BBOARD command adds the specified mailbox name to
the list of "active" or "subscribed" bulletin boards as
returned by the FIND BBOARDS command. This command returns an
OK response only if the subscription is successful.
If SUBSCRIBE BBOARD is implemented then FIND BBOARDS must also
be implemented, and FIND ALL.BBOARDS should be implemented.
tag UNSUBSCRIBE BBOARD bboard
The UNSUBSCRIBE BBOARD command removes the specified mailbox
name from the list of "active" or "subscribed" bulletin boards
as returned by the FIND BBOARDS command. This command returns
an OK response only if the unsubscription is successful.
If UNSUBSCRIBE BBOARD is implemented then SUBSCRIBE BBOARDS and
FIND BBOARDS must also be implemented, and FIND ALL.BBOARDS
should be implemented.
tag SELECT mailbox
This command select a particular mailbox. The server must check
that the user is permitted read access to this mailbox. Before
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returning an OK to the client, the server must send the following
unsolicited data to the client:
FLAGS mailbox's defined flags
<n> EXISTS the number of messages in the mailbox
<n> RECENT the number of new messages in the mailbox
in order to define the initial state of the mailbox at the client.
Multiple selection commands are permitted in a session, in which
case the previous mailbox is automatically deselected when a new
selection is made.
The mailbox name INBOX is a special name reserved to mean "the
primary mailbox for this user on this server". The format of
other mailbox names is implementation dependent.
The text of an OK response to the SELECT command should begin with
either "[READ-ONLY]" or "[READ-WRITE]" to show the mailbox's
access status.
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. EXAMINE has the
same mailbox namespace as SELECT.
Server support for EXAMINE is optional.
tag BBOARD mailbox
The BBOARD command is similar to SELECT, and returns the same
output. Its argument is a shared mailbox (bulletin board) name
instead of an ordinary mailbox. There is no requirement that the
namespace for BBOARD be the same as that for SELECT and EXAMINE.
BBOARD also differs from EXAMINE in that it may allow changes
(e.g. marking a message as seen or deleted) to a mailbox; the
exact handling of this is implementation dependent.
Server support for BBOARD is optional.
tag CHECK
The CHECK command requests a checkpoint of the mailbox. CHECK may
cause an operation which may take a non-instantaneous amount of
real-time to complete. The exact meaning of a checkpoint is
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implementation-dependent. Possible interpretations include
forcing a 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.
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 you will receive 5 unsolicited EXPUNGE responses
for message 5.
tag COPY sequence mailbox
The COPY command copies the specified message(s) to the specified
destination mailbox.
If the destination mailbox does not exist, a server that supports
the CREATE command 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, prior to
the tagged NO response, send an unsolicited OK response with text
beginning with the string "[TRYCREATE]". 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, IMAP2bis
compliant server implementations MUST restore the destination
mailbox to its state prior to the COPY.
EXAMPLE: A003 COPY 2:4 MEETING
copies messages 2, 3, and 4 to mailbox "MEETING".
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tag APPEND mailbox string
The APPEND command appends the string, which is in the format of
an RFC 822 message, to the specified mailbox. If the append is
unsuccessful for any reason the mailbox must be restored to its
state prior to the append; no partial appending is permitted. If
the mailbox is presently selected, the normal new mail actions
should occur.
If the destination mailbox does not exist, a server MUST return an
error, and MUST NOT automatically create the mailbox. Unless it
is certain that the destination mailbox can not be created, the
server MUST, prior to the tagged NO response, send an unsolicited
OK response with text beginning with the string "[TRYCREATE]".
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 RFC 821 (SMTP)
envelope information.
Server support for APPEND is optional.
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 The MIME body structure of the message. The
body is computed by the server by parsing the
MIME header lines.
Server support for this data item is optional.
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.
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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
itself, only its individual members.
A part of type MESSAGE also has nested parts,
which are the parts of the MESSAGE part's body.
Every message has at least one part.
EXAMPLE: Here is a very complex message with its
associated section specifications.
1 TEXT/PLAIN
2 APPLICATION/OCTET-STREAM
3 MESSAGE/RFC822
3.1 TEXT/PLAIN
3.2 APPLICATION/OCTET-STREAM
MULTIPART/MIXED
4.1 IMAGE/GIF
4.2 MESSAGE/RFC822
4.2.1 TEXT/PLAIN
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).
The \SEEN flag is implicitly set; if this causes
the flags to change they should be included as
part of the fetch results.
Server support for this data item is optional.
ENVELOPE The envelope of the message. The envelope is
computed by the server by parsing the RFC 822
header into the component parts, defaulting
various fields as necessary.
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)
Server support for this data item is optional.
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INTERNALDATE The date and time the message was written to
the mailbox.
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.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.
This is used to obtain a filtered header
containing only desired header lines.
Server support for this data item is optional.
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.
This is used to obtain a filtered header not
containing undesired header lines.
Server support for this data item is optional.
RFC822.SIZE The number of characters in the message as
expressed in RFC 822 format.
RFC822.TEXT The text body of the message, omitting the
RFC 822 header. The \SEEN flag is implicitly
set; if this causes the flags to change they
should be included as part of the fetch results.
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UID The unique identifier for the message.
Server support for this data item is optional.
EXAMPLES:
A003 FETCH 2:4 ALL
fetches the flags, internal date, RFC 822 size, and envelope
for messages 2, 3, and 4.
A004 FETCH 3 RFC822
fetches the RFC 822 representation for message 3.
A005 FETCH 4 (FLAGS RFC822.HEADER)
fetches the flags and RFC 822 format header for message 4.
tag PARTIAL msgno data start_byte byte_count
The PARTIAL command is equivalent to the associated FETCH command,
with the added functionality that only the specified number of
bytes, beginning at the specified starting byte, are returned.
Note that only a single message can be fetched at a time. The
first byte of a message, and hence the minimum for the starting
byte, is byte 1.
The following FETCH items are valid data for PARTIAL: RFC822,
RFC822.HEADER, RFC822.TEXT, and BODY[section].
Any partial fetch which attempts to read beyond the end of the
text is truncated as appropriate. If the starting byte is beyond
the end of the text, an empty string is returned.
The data is 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
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 that data.
Note that when partial fetching it is possible to break in the
middle of a a line or a criticial 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 bytes 1 through 10000 breaks in
an awkward place, it is permitted to continue with a partial fetch
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of 9987 through 19987, etc.
The handling of the \SEEN flag is the same as with the FETCH
command.
Server support for PARTIAL is optional.
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.
UID commands
These commands use unique identifiers instead of message numbers in
their arguments to reference a particular message or range of
messages.
Sequence ranges are permitted; note however that there is no
guarantee that unique identifiers be contiguous. A non-existant
unique identifier within a sequence range is ignored without any
error message generated.
Because of the potential for ambiguity, the UID command does not
change responses. That is, the number after the "*" in an
unsolicited FETCH response is a message number, not a unique
identifier. However, servers MUST implicitly include UID as part of
any FETCH response caused by a UID command, regardless of whether or
not UID was specified.
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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
tag UID AFTER uniqueid
The UID AFTER command is used to determine what unique
identifiers exist that are greater than the specified unique
identifier. It returns unsolicited FETCH responses for each
such message.
For example, if the specified unique identifier refers to
message 572 in a mailbox with 613 messages, the results
returned are equivalent to doing "FETCH 573:613 UID".
tag UID BEFORE uniqueid
The UID BEFORE command is used to determine what unique
identifiers exist that are less than the specified unique
identifier. It returns unsolicited FETCH responses for each
such message.
For example, if the specified unique identifier refers to
message 572 in a mailbox with 613 messages, the results
returned are equivalent to doing "FETCH 1:571 UID".
tag UID COPY sequence mailbox
The UID COPY command is identical to the COPY command, with the
exception that the numbers used in the sequence are unique
identifiers instead of message numbers.
tag UID FETCH sequence data
The UID FETCH command is identical to the FETCH command, with
the exception that the numbers used in the sequence are unique
identifiers instead of message numbers.
tag UID STORE sequence data value
The UID STORE command is identical to the STORE command, with
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the exception that the numbers used in the sequence are unique
identifiers instead of message numbers.
tag SEARCH search_criteria
The SEARCH command searches the mailbox for messages that match
the given set of criteria. The unsolicited SEARCH <1#number>
response from the server is a list of messages that express the
intersection (AND function) of all the messages which 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.
In all search criteria which use strings, a message matches the
criteria if the string is a substring of that field. The matching
is case-independent except as noted below.
The string may be in RFC 1342 format to express a character set
other than US-ASCII. In this case, the criteria matches if the
string matches the character set and characters of a substring of
that field. A server implementation may omit case-independent
matching on RFC 1342 strings.
The currently defined search criteria are:
ALL All messages in the mailbox; the default
initial criterion for ANDing.
ANSWERED Messages with the \ANSWERED flag set.
BCC istring Messages which contain the specified string
in the envelope's BCC field.
BEFORE date Messages whose internal date is earlier than
the specified date.
BODY istring Messages which contain the specified string
in the body of the message.
CC istring Messages which contain the specified string
in the envelope's CC field.
DELETED Messages with the \DELETED flag set.
FLAGGED Messages with the \FLAGGED flag set.
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FROM istring Messages which contain the specified string
in the envelope's FROM field.
KEYWORD flag Messages with the specified flag set.
NEW Messages which have the \RECENT flag set but
not the \SEEN flag. This is functionally
equivalent to "RECENT UNSEEN".
OLD Messages which do not have the \RECENT flag
set.
ON date Messages whose internal date is within the
specified date.
RECENT Messages which have the \RECENT flag set.
SEEN Messages which have the \SEEN flag set.
SINCE date Messages whose internal date is later than
the specified date.
SUBJECT istring Messages which contain the specified string
in the envelope's SUBJECT field.
TEXT istring Messages which contain the specified string.
TO istring Messages which contain the specified string in
the envelope's TO field.
UNANSWERED Messages which do not have the \ANSWERED flag
set.
UNDELETED Messages which do not have the \DELETED flag
set.
UNFLAGGED Messages which do not have the \FLAGGED flag
set.
UNKEYWORD flag Messages which do not have the specified flag
set.
UNSEEN Messages which do not have the \SEEN flag set.
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Responses
The first group of responses complete a request, and indicate whether
or not the command was successful. The response text is a line of
human readable text, optionally prefixed by an atom inside square
brackets that conveys a special information token between cooperating
servers and clients.
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.
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.
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
LOGIN command that uses Kerberos authentication.
Other special information tokens defined by particular client or
server implementations should be prefixed with an "X" until such time
as they are added to a revision of this protocol.
tag OK response_text
This response identifies successful completion of the command with
that tag. The response text may be useful in a protocol telemetry
log for debugging purposes.
tag NO response_text
This response identifies unsuccessful completion of the command
with that tag. The text is a line of human-readable text that
probably should be displayed to the user in an error report by the
client.
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tag BAD response_text
This response identifies faulty protocol received from the client;
The 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.
The second group of responses convey human-readable information. The
response text is a line of human readable text, optionally prefixed
by an atom inside square brackets that conveys special information
between cooperating servers and clients.
* PREAUTH response_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 response_text
This response identifies 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.
* NO response_text
This response identifies 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.
* BAD response_text
This response identifies a serious error message from the server.
It does not indicate completion of any request, nor is it
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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.
* BYE text
This response identifies 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 at the present time.
The third group of responses convey data about the mailbox or
messages inside the mailbox. This is how message data is transmitted
from the server to the client.
* MAILBOX mstring
This response occurs as a result of a FIND command for MAILBOXES
and ALL.MAILBOXES. The string is a mailbox name that matches the
pattern in the command.
* BBOARD mstring
This response occurs as a result of a FIND command for BBOARDS and
ALL.BBOARDS. The string is a bulletin board name that matches the
pattern in the command.
* 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, e.g., "SEARCH 2 3 6".
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* FLAGS flag_list
This response occurs as a result of a selection command (SELECT,
BBOARD, and EXAMINE). 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.
* number message_data
This response occurs as a result of any command when a mailbox is
selected. The message_data is one of the following:
EXISTS The specified number of messages exists in the mailbox.
RECENT The specified number of messages have arrived since the
previous time this mailbox was read.
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 principle means by which data about a
message is returned to the client. The data is in a
S-expression form, and consists of a sequence of pairs
of data item name and their values. The current data
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items are:
BODY An S-expression format list that describes the
body of a message. The body 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
nesting. In this case, instead of a body type
as the first element of the list there is a nested
body. The last element of the list is the multipart
subtype (mixed, digest, parallel, alternative, etc.).
The basic fields of a non-multipart body part are in
the following order:
body type - an atom 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 that
body in bytes. Note that this size is the
size in its transfer encoding and not the
resulting size.
A body type of type MESSAGE and subtype RFC822
contains, immediately after the basic fields,
the envelope of the encapsulated message, its
body structure, and its size in text lines.
A body type of type TEXT contains, immediately
after the basic fields, the size of the body
in text lines.
BODY[section] A string expressing the body contents of the
specified section. This string is transmitted
as 7-bit US-ASCII, and interpreted according to
the content transfer encoding and the body type
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and subtype.
Note that 7-bit US-ASCII is the transport format
and NOT necessarily the byte size or character
set of the result. The resulting data may be
8-bit; it may also be some other character set
or binary.
ENVELOPE An S-expression format list that describes the
envelope of a message. The envelope is computed
by the server by parsing the RFC 822 header into
the component parts, defaulting various fields
as necessary.
The fields of the envelope are in the following
order: date, subject, from, sender, reply-to, to,
cc, bcc, in-reply-to, and message-id. The date,
subject, in-reply-to, and message-id fields are
strings. The from, sender, reply-to, to, cc,
and bcc fields are lists of addresses.
An address is an S-expression format list that
describes an electronic mail address. The fields
of an address are in the following order:
personal name, source-route (a.k.a. the
at-domain-list in SMTP), mailbox name, and
host name.
Any field of an envelope or address 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:
\RECENT Message arrived since the
previous time this mailbox
was read
\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
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INTERNALDATE A string containing the date and time the
message was written to the mailbox.
RFC822 A string expressing the message in RFC 822
format.
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 string expressing the unique identifier
of the message.
COPY Obsolete. New server implementations MUST NOT transmit
this response. Client implementations SHOULD ignore
this response (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.
The final group of responses contains a single, special purpose
response.
+ text
This response identifies 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).
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The purpose of this command is to solve a synchronization problem
which can occur if one of the strings in a command is a literal.
This may occur when logging in (if the password contains "funny"
characters) and especially with the APPEND command (since the
message sent 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. Nor is any necessary. Any
synchronization problems in this direction would be caused by a
bug in the client or server.
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Sample IMAP2bis session
The following is a transcript of an IMAP2bis session. One very long
long line in this sample is broken for editorial clarity.
S: * OK IMAP2bis Service 7.2(62) at Thu, 29 Jul 1993 21:34:23 -0700 (PDT)
C: a001 login mrc secret
S: a001 OK LOGIN completed
C: a002 select inbox
S: * 18 EXISTS
S: * FLAGS (\Answered \Flagged \Deleted \Seen)
S: * 0 RECENT
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)"
"IMAP2bis WG mtg summary and minutes" (("Terry Gray" NIL "gray"
"cac.washington.edu")) ((NIL NIL "owner-imap" "cac.washington.edu"))
(("Terry Gray" NIL "gray" "cac.washington.edu")) ((NIL NIL "imap"
"cac.washington.edu")) ((NIL NIL "minutes" "CNRI.Reston.VA.US")
("John C Klensin" NIL "KLENSIN" "INFOODS.MIT.EDU")("Erik Huizer"
NIL "Erik.Huizer" "SURFnet.nl")) NIL NIL
"<Pine.3.84.9307140123.B27397-0100000@shiva2.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 {485}
S: Date: Wed, 14 Jul 1993 02:23:25 -0700 (PDT)
S: From: Terry Gray <gray@cac.washington.edu>
S: Reply-To: Terry Gray <gray@cac.washington.edu>
S: Subject: IMAP2bis WG mtg summary and minutes
S: To: imap@cac.washington.edu
S: Cc: minutes@CNRI.Reston.VA.US,
S: John C Klensin <KLENSIN@INFOODS.MIT.EDU>,
S: Erik Huizer <Erik.Huizer@SURFnet.nl>
S: Message-Id:
S: <Pine.3.84.9307140123.B27397-0100000@shiva2.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 IMAP2bis server terminating connection
S: a006 OK LOGOUT completed
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Design Discussion
IMAP2bis is a textual protocol. The use of MIME encoding in IMAP2bis
makes it possible to support 8-bit textual and binary mail.
IMAP2bis implementations MAY transmit 8-bit 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.
For compatibility with the 7-bit world, servers MUST NOT transmit 8-
bit characters in RFC822.HEADER fetch results, and SHOULD NOT
transmit 8-bit characters in RFC822.TEXT (and by extension RFC822)
fetch results.
Because 8-bit characters are permitted in the argument to APPEND, a
server in the 7-bit world MUST, if it implements the APPEND command,
be able to make a reversible conversion of the 8-bit APPEND data to
7-bit data using MIME.
Although a BINARY body encoding is defined, IMAP2bis does not permit
unencoded binary strings. A "binary string" is any string with NUL
characters; a string with an excessive number 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 prior to transmitting it with IMAP2bis.
When operating in the online model, an IMAP2bis 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 unsolicited 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 in the course of 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
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while there is no command in progress, this practice SHOULD NOT be
followed due to flow control considerations. It can cause an
incautious implementation to deadlock. A deadlock is avoided if
either of the following conditions are true: (1) with the exception
of the greeting, the server never sends responses while there is no
command in progress; (2) the client always reads responses from the
IMAP2bis connection regardless of whether or not a command is in
progress (or even if it is transmitting the command). The latter
condition generally requires a multi-threading client implementation,
which may not always be feasible.
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, or even act as a poll for updates.
It is frequently asked why there is no message posting capabilities
in IMAP2bis. 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 capability in IMAP2bis would need, at a minimum, to
provide equivalent capabilities.
At the time of the writing of this document, an extensive set of
extensions to SMTP is in the Internet standards process. Should
those extensions become an Internet Standard it would be necessary to
revise IMAP2bis again to provide corresponding capabilities, if a
message posting facility was included in IMAP2bis. 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 IMAP2bis occurs when
a (very) remote server is used. It is unlikely that a client will
support multiple means of posting a message; it adds excessive size
and complexity which can not be afforded, particularly on smaller
machines. Consider a client connecting to an IMAP2bis server over an
interactive satellite link to a foreign country. A local message
posting (SMTP) server is available which uses a lower-cost batched
link. In this case, the use of the interactive link would be
wasteful and costly.
Message posting to IMAP2bis 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
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authentication credentials in a mail message (as a "From" or "Sender"
address) may be a serious security breach of greater significance
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 authenticaton 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. It is for this reason that public-key based
message authentication systems such as Privacy Enhanced Mail are
presently under development.
IMAP2bis does not address problems with multiple IMAP2bis 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 (Interactive Mail
Support Protocol) being developed at CMU.
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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 IMAP2bis
protocol are case-insensitive. For example, "LOGIN", "login" and
"lOgIn" are all valid as the login command.
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 ::= nil / string
addr_host ::= nil / string
addr_mailbox ::= nil / string
addr_name ::= nil / string
append ::= "APPEND" SPACE mailbox SPACE string
astring ::= atom / string
atom ::= 1*<any CHAR except specials, SPACE, and CTLs>
bboard ::= "BBOARD" SPACE mailbox_bboard
body ::= "(" body_structure ")"
body_basic ::= body_type SPACE body_subtype SPACE body_parameter
SPACE body_id SPACE body_description SPACE
body_encoding SPACE body_size_byte
body_description::= nil / string
body_encoding ::= "7BIT" / "8BIT" / "BINARY" / "BASE64"/
"QUOTEDPRINTABLE" / atom
body_id ::= nil / string
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body_msg ::= body_basic SPACE envelope SPACE body
body_parameter ::= nil / "(" 1#(string string) ")"
body_section ::= number / number "." body_section
body_size_byte ::= number
body_size_line ::= number
body_structure ::= body_basic / body_msg / body_text /
"(" body_structure ")" SPACE body_subtype
body_subtype ::= nil / string
body_text ::= body_basic SPACE body_size_line
body_type ::= "TEXT" / "MULTIPART" / "MESSAGE" / "APPLICATION" /
"AUDIO" / "IMAGE" / "VIDEO" / atom
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
CRLF ::= CR LF
CTL ::= <any ASCII control character and DEL, 0x00 - 0x1f, 0x7f>
date ::= date_new / date_old
date_day ::= 1*2DIGIT
;; day of month
date_month ::= "Jan" / "Feb" / "Mar" / "Apr" / "May" / "Jun" /
"Jul" / "Aug" / "Sep" / "Oct" / "Nov" / "Dec"
date_new ::= date_day "-" date_month "-" date_year
date_old ::= date_day "-" date_month "-" date_year_old
;; Obsolete
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date_year ::= 4DIGIT / date_year_old
date_year_old ::= 2DIGIT
;; Obsolete, (year - 1900)
date_time ::= date_time_new / date_time_old
date_time_new ::= date_new SPACE time SPACE zone
date_time_old ::= date_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 ::= string
env_from ::= nil / "(" 1*address ")"
env_in-reply-to ::= nil / string
env_message-id ::= nil / string
env_reply-to ::= nil / "(" 1*address ")"
env_sender ::= nil / "(" 1*address ")"
env_subject ::= nil / string
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_other / fetch_att_text
fetch_att_other ::= "BODY" / "ENVELOPE" / "FLAGS" / "INTERNALDATE" /
"RFC822.SIZE" / "UID"
fetch_att_text ::= "BODY[" body_section "]" / "RFC822" /
"RFC822.HEADER" /
"RFC822.HEADER.LINES" SPACE header_line_list /
"RFC822.HEADER.LINES.NOT" SPACE header_line_list /
"RFC822.TEXT"
find ::= find_mailbox / find_bboard
find_bboard ::= find_bboards / find_boards_all
find_bboards ::= "FIND" SPACE "BBOARDS" SPACE find_pattern
find_bboards_all
::= "FIND" SPACE "ALL.BBOARDS" SPACE find_pattern
find_mailbox ::= find_mailboxes / find_mailboxes_all
find_mailboxes ::= "FIND" SPACE "MAILBOXES" SPACE find_pattern
find_mailboxes_all
::= "FIND" SPACE "ALL.MAILBOXES" SPACE find_pattern
find_pattern ::= astring
;; includes find_wildcards
find_wildcards ::= "%" / "?" / "*"
flag ::= user_flag / system_flag
flag_list ::= "(" 1#flag ")"
flags ::= 1#flag / flag_list
greeting ::= "*" SPACE (human_data / "PREAUTH" SPACE response_text)
header_line ::= string
header_line_list ::= "(" 1#header_line ")"
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human_data ::= "OK" SPACE response_text / "NO" SPACE response_text /
"BAD" SPACE response_text / "BYE" SPACE resonse_text
inbox ::= "INBOX"
;; case-independent, but SHOULD be upper-case
istring ::= astring
;; possible RFC 1342 format data
kerberos_authenticator
::= 1*DIGIT_HEX
kerberos_response
::= 1*DIGIT_HEX
LF ::= <ASCII LF, line feed, 0x0A>
literal ::= "{" number "}" CRLF 1*CHAR8
;; The number represents the number of CHAR8 octets.
login ::= "LOGIN" SPACE userid SPACE password
logout ::= "LOGOUT"
mailbox ::= inbox / mailbox_other
mailbox_bboard ::= astring
;; May not be INBOX (in any case). Should not
;; include find_wildcards. May be case-dependent
;; as a function of server implementation. May
;; be a different namespace from mailbox_other.
mailbox_other ::= astring
;; May not be INBOX (in any case). Should not
;; include find_wildcards. May be case-dependent
;; as a function of server implementation
mailbox_data ::= "MAILBOX" SPACE mstring / "BBOARD" SPACE mstring /
"SEARCH" SPACE 1#number / "FLAGS" SPACE flag_list
message_data ::= number SPACE (msg_exists / msg_recent / msg_expunge /
msg_fetch / msg_obsolete)
msg_copy ::= "COPY"
;; Obsolete
msg_exists ::= "EXISTS"
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msg_expunge ::= "EXPUNGE"
msg_fetch ::= "FETCH" SPACE "(" 1#("BODY" SPACE body /
"BODY[" body_section "]" string /
"ENVELOPE" SPACE envelope /
"FLAGS" SPACE "(" 1#(recent_flag / flag) ")" /
"INTERNALDATE" SPACE date_time /
"RFC822" SPACE string /
"RFC822.HEADER" SPACE string /
"RFC822.SIZE" SPACE number /
"RFC822.TEXT" SPACE string /
"UID" SPACE uniqueid) ")"
msg_obsolete ::= msg_copy / msg_store
;; Obsolete unsolicited data responses
msg_recent ::= "RECENT"
msg_store ::= "STORE" SPACE "(" 1#("FLAGS" SPACE
"(" 1#(recent_flag / flag) "))"
;; Obsolete
mstring ::= text_line
;; Represents a mailbox
nil ::= "NIL"
noop ::= "NOOP"
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 ::= <"> / "%" / "\"
quoted_string ::= <"> *QCHAR <">
recent_flag ::= "\RECENT"
ready ::= "+" SPACE response_text
rename ::= "RENAME" SPACE mailbox SPACE mailbox_other
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request ::= tag SPACE (noop / login / logout / create /
delete / rename / find / subscribe / unsubscribe /
select / examine / bboard / check / expunge /
copy / append / fetch / partial / store / uid /
search / x_command ) CRLF
response ::= tag SPACE ("OK" / "NO" / "BAD") SPACE response_text CRLF
response_code ::= "PARSE" / "READ-ONLY" / "READ-WRITE" / "TRYCREATE" /
"X" atom / kerberos_response
response_text ::= ("[" response_code "]" text_line) / text_line
search ::= "SEARCH" SPACE 1#("ALL" / "ANSWERED" /
"BCC" SPACE istring / "BEFORE" SPACE day /
"BODY" SPACE istring / "CC" SPACE istring / "DELETED" /
"FLAGGED" / "KEYWORD" SPACE atom / "NEW" / "OLD" /
"ON" SPACE day / "RECENT" / "SEEN" /
"SINCE" SPACE day / "TEXT" SPACE istring /
"TO" SPACE istring / "UNANSWERED" / "UNDELETED" /
"UNFLAGGED" / "UNKEYWORD" / "UNSEEN")
select ::= "SELECT" SPACE mailbox
sequence ::= number / (sequence "," sequence) / (number ":" number)
;; 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: 2,4:7,9,12:15 is 2,4,5,6,7,9,12,13,14,15
SPACE ::= <ASCII SP, space, 0x20>
specials ::= "{" / qspecials
store ::= "STORE" SPACE sequence SPACE store_att
store_att ::= "+FLAGS" SPACE flags / "-FLAGS" SPACE flags /
"FLAGS" SPACE flags
string ::= quoted_string / literal
subscribe ::= subscribe_mailbox / subscribe_bboard
subscribe_bboard
::= "SUBSCRIBE" SPACE "BBOARD" SPACE mailbox_bboard
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subscribe_mailbox
::= "SUBSCRIBE" SPACE "MAILBOX" SPACE mailbox
system_flag ::= "\ANSWERED" / "\FLAGGED" / "\DELETED" / "\SEEN"
tag ::= atom
text_line ::= 1*<any CHAR except CR and LF>
time ::= 2DIGIT ":" 2DIGIT ":" 2DIGIT
;; hours minutes seconds
uid ::= "UID" SPACE (uid_after / uid_before / copy /
fetch / store)
;; uniqueids used instead of message numbers
uid_after ::= "AFTER" SPACE uniqueid
uid_before ::= "BEFORE" SPACE uniqueid
uniqueid ::= number
;;; strictly ascending
unsolicited ::= "*" SPACE (human_data / mailbox_data /
message_data) CRLF
unsubscribe ::= unsubscribe_mailbox / unsubscribe_bboard
unsubscribe_bboard
::= "UNSUBSCRIBE" SPACE "BBOARD" SPACE mailbox_bboard
unsubscribe_mailbox
::= "UNSUBSCRIBE" SPACE "MAILBOX" SPACE mailbox_mailbox
userid ::= astring
user_flag ::= atom
x_command ::= "X" atom <optional arguments>
;; experimental expansion commands
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".
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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: *<unsolicited>
Server: response
>
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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.
The present protocol would not have come into existance without the
assistance of the IETF IMAP2 working group, and in particular Ned
Freed, John Myers, Chris Newman, and Adam Treister.
Any mistakes, flaws, or sins of omission in this IMAP2bis 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 for the purpose of accessing 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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