RFC1176

Network Working Group M. Crispin Request for Comments: 1176 Washington Obsoletes: RFC 1064 August 1990

             INTERACTIVE MAIL ACCESS PROTOCOL - VERSION 2

Status of this Memo

  This RFC suggests a method for personal computers and workstations to
  dynamically access mail from a mailbox server ("repository").  It
  obosoletes RFC 1064.  This RFC specifies an Experimental Protocol for
  the Internet community.  Discussion and suggestions for improvement
  are requested.  Please refer to the current edition of the "IAB
  Official Protocol Standards" for the standardization state and status
  of this protocol.  Distribution of this memo is unlimited.

Introduction

  The intent of the Interactive Mail Access Protocol, Version 2 (IMAP2)
  is to allow a workstation, personal computer, or similar small
  machine to access electronic mail from a mailbox server.  Since the
  distinction between personal computers and workstations is blurring
  over time, it is desirable to have a single solution that addresses
  the need in a general fashion.  IMAP2 is the "glue" of a distributed
  electronic mail system consisting of a family of client and server
  implementations on a wide variety of platforms, from small single-
  tasking personal computing engines to complex multi-user timesharing
  systems.

  Although different in many ways from the Post Office Protocols (POP2
  and POP3, hereafter referred to collectively as "POP") described in
  RFC 937 and RFC 1081, IMAP2 may be thought of as a functional
  superset of these.  RFC 937 was used as a model for this RFC.  There
  was a cognizant reason for this; POP deals with a similar problem,
  albeit with a less comprehensive solution, and it was desirable to
  offer a basis for comparison.

  Like POP, IMAP2 specifies a means of accessing stored mail and not of
  posting mail; this function is handled by a mail transfer protocol
  such as SMTP (RFC 821).

  This protocol assumes a reliable data stream such as provided by TCP
  or any similar protocol.  When TCP is used, the IMAP2 server listens
  on port 143.

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RFC 1176 IMAP2 August 1990

System Model and Philosophy

  Electronic mail is a primary means of communication for the widely
  spread Internet community.  The advent of distributed personal
  computers and workstations has forced a significant rethinking of the
  mechanisms employed to manage electronic mail.  With mainframes, each
  user tends to receive and process mail at the computer he uses most
  of the time, his "primary host".  The first inclination of many users
  when an independent workstation is placed in front of them is to
  begin receiving mail at the workstation, and many vendors have
  implemented facilities to do this.  However, this approach has
  several disadvantages:

     (1) Personal computers and many workstations have a software
     design that gives full control of all aspects of the system to the
     user at the console.  As a result, background tasks such as
     receiving mail may not run for long periods of time; either
     because the user is asking to use all the machine's resources, or
     because the user has (perhaps accidentally) manipulated the
     environment in such a way that it prevents mail reception.  In
     many personal computers, the operating system is single-tasking
     and this is the only mode of operation.  Any of these conditions
     could lead to repeated failed delivery attempts by outside agents.

     (2) The hardware failure of a single machine can keep its user
     "off the air" for a considerable time, since repair of individual
     units may be delayed.  Given the growing number of personal
     computers and workstations spread throughout office environments,
     quick repair of such systems is not assured.  On the other hand, a
     central mainframe is generally repaired soon after failure.

     (3) Personal computers and workstations are often not backed up
     with as much diligence as a central mainframe, if at all.

     (4) It is more difficult to keep track of mailing addresses when
     each person is associated with a distinct machine.  Consider the
     difficulty in keeping track of many postal addresses or phone
     numbers, particularly if there was no single address or phone
     number for an organization through which you could reach any
     person in that organization.  Traditionally, electronic mail on
     the ARPANET involved remembering a name and one of several "hosts"
     (machines) whose name reflected the organization in which the
     individual worked.  This was suitable at a time when most
     organizations had only one central host.  It is less satisfactory
     today unless the concept of a host is changed to refer to an
     organizational entity and not a particular machine.

     (5) It is difficult to keep a multitude of heterogeneous machines

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RFC 1176 IMAP2 August 1990

     working properly with complex mailing protocols, making it
     difficult to move forward as progress is made in electronic
     communication and as new standards emerge.  Each system has to
     worry about receiving incoming mail, routing and delivering
     outgoing mail, formatting, storing, and providing for the
     stability of mailboxes over a variety of possible filing and
     mailing protocols.

  Consequently, while a personal computer or workstation may be viewed
  as an Internet host in the sense that it implements TCP/IP, it should
  not be viewed as the entity that contains the user's mailbox.
  Instead, a mail server machine ("server", sometimes called a
  "repository") should hold the mailbox, and the personal computer or
  workstation (hereafter referred to as a "client") should access the
  mailbox via mail transactions.

  Because the mail server machine is isolated from direct user
  manipulation, it should achieve high software reliability easily,
  and, as a shared resource, it should also achieve high hardware
  reliability, perhaps through redundancy.  The mail server may be
  accessed from arbitrary locations, allowing users to read mail across
  campus, town, or country using commonly available clients.
  Furthermore, the same user may access his mailbox from different
  clients at different times, and multiple users may access the same
  mailbox simultaneously.

  The mail server acts an an interface among users, data storage, and
  other mailers.  A mail access protocol retrieves messages, accesss
  and changes properties of messages, and otherwise manages mailboxes.
  This differs from some approaches (e.g., Unix mail via NFS) in that
  the mail access protocol is used for all message manipulations,
  isolating the user and the client from all knowledge of how the data
  storage is used.  This means that the mail server can use the data
  storage in whatever way is most efficient to organize the mail in
  that particular environment, without having to worry about storage
  representation compatibility across different machines.

  A mail access protocol further differs in that it transmits
  information only on demand.  A well-designed mail access protocol
  requires considerably less network traffic than Unix mail via NFS,
  particularly when the mail file is large.  The result is that a mail
  access protocol can scale well to situations of large mailboxes or
  networks with high latency or low speed.

  In defining a mail access protocol, it is important to keep in mind
  that the client and server form a macrosystem, in which it should be
  possible to exploit the strong points of both while compensating for
  each other's weaknesses.  Furthermore, it is desirable to allow for a

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RFC 1176 IMAP2 August 1990

  growth path beyond the hoary text-only RFC 822 protocol, specifically
  in the area of attachments and multi-media mail, to ease the eventual
  transition to ISO solutions.

  Unlike POP, IMAP2 has extensive features for remote searching and
  parsing of messages on the server.  A free text search (optionally
  with other searching) can be made in the entire mailbox by the server
  and the results made available to the client without the client
  having to transfer the entire mailbox and searching itself.  Since
  remote parsing of a message into a structured (and standard format)
  "envelope" is available, a client can display envelope information
  and implement commands such as REPLY without having any understanding
  of how to parse RFC 822, etc. headers.  The effect of this is
  twofold: it further improves the ability to scale well in instances
  where network traffic must be reduced, and it reduces the complexity
  of the client program.

  Additionally, IMAP2 offers several facilities for managing individual
  message state and the mailbox as a whole beyond the simple "delete
  message" functionality of POP.  Another benefit of IMAP2 is the use
  of tagged responses to reduce the possibility of synchronization
  errors and the concept of state on the client (a "local cache") that
  the server may update without explicit request by the client.  These
  concepts and how they are used are explained under "Implementation
  Discussion" below.

  In spite of this functional richness, IMAP2 is a small protocol.
  Although servers should implement the full set of IMAP2 functions, a
  simple client can be written that uses IMAP2 in much the way as a POP
  client.

  A related protocol to POP and IMAP2 is the DMSP protocol of PCMAIL
  (RFC 1056).  IMAP2 differs from DMSP more fundamentally, reflecting a
  differing architecture from PCMAIL.  PCMAIL is either an online
  ("interactive mode"), or offline ("batch mode") system with long-term
  shared state.  Some POP based systems are also offline; in such
  systems, since there is no long-term shared state POP is little more
  than a download mechanism of the "mail file" to the client.  IMAP2-
  based software is primarily an online system in which real-time and
  simultaneous mail access were considered important.

  In PCMAIL, there is a long-term client/server relationship in which
  some mailbox state is preserved on the client.  There is a
  registration of clients used by a particular user, and the client
  keeps a set of "descriptors" for each message that summarize the
  message.  The server and client synchronize their states when the
  DMSP connection starts up, and, if a client has not accessed the
  server for a while, the client does a complete reset (reload) of its

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  state from the server.

  In IMAP2-based software, the client/server relationship lasts only
  for the duration of the TCP connection.  All mailbox state is
  maintained on the server.  There is no registration of clients.  The
  function of a descriptor is handled by a structured representation of
  the message "envelope" as noted above.  There is no client/server
  synchronization since the client does not remember state between
  IMAP2 connections.  This is not a problem since in general the client
  never needs the entire state of the mailbox in a single session,
  therefore there isn't much overhead in fetching the state information
  that is needed as it is needed.

  There are also some functional differences between IMAP2 and DMSP.
  DMSP has functions for sending messages, printing messages, listing
  mailboxes, and changing passwords; these are done outside IMAP2.
  DMSP has 16 binary flags of which 8 are defined by the system.  IMAP2
  has flag names; there are currently 5 defined system flag names and a
  facility for some number (30 in the current implementations) of user
  flag names.  IMAP2 has a sophisticated message search facility in the
  server to identify interesting messages based on dates, addresses,
  flag status, or textual contents without compelling the client to
  fetch this data for every message.

  It was felt that maintaining state on the client is advantageous only
  in those cases where the client is only used by a single user, or if
  there is some means on the client to restrict access to another
  user's data.  It can be a serious disadvantage in an environment in
  which multiple users routinely use the same client, the same user
  routinely uses different clients, and where there are no access
  restrictions on the client.  It was also observed that most user mail
  access is to a small set of "interesting" messages, which were either
  new mail or mail based on some user-selected criteria.  Consequently,
  IMAP2 was designed to easily identify those "interesting" messages so
  that the client could fetch the state of those messages and not those
  that were not "interesting".

The Protocol

  The IMAP2 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 this 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 "*".  There

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RFC 1176 IMAP2 August 1990

  is another special reserved tag, "+", discussed below.

  The server must be listening for a connection.  When a connection is
  opened the server sends an unsolicited OK response as a greeting
  message and then waits for commands.

  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 and is not under any obligation to wait for a server
  response to this command before sending another command, within the
  constraints of TCP flow control.  When commands are received the
  server acts on them and responds with command responses, often
  interspersed with data.  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.

  Although all known IMAP2 servers at the time of this writing process
  commands to completion before processing the next command, it is not
  required that a server do so.  However, many commands can affect the
  results of other commands, creating processing-order dependencies
  (or, for SEARCH and FIND, ambiguities about which data is associated
  with which command).  All implementations that operate in a non-
  lockstep fashion must recognize such dependencies and defer or
  synchronize execution as necessary.  In general, such multi-
  processing is limited to consecutive FETCH commands.

  Generally, the first command from the client is a LOGIN command with
  user name and password arguments to establish identity and access
  authorization, unless this has already been accomplished through
  other means, e.g. Kerberos.  Until identity and access authorization
  have been established, no operations other than LOGIN or LOGOUT are
  permitted.

  Once identity and authorization have been established, the client
  must send a SELECT command to access the desired mailbox; no mailbox
  is selected by default.  SELECT's argument is 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.  On a successful SELECT, 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 SELECT command.

  The client reads mailbox information with FETCH commands.  The actual

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  data is transmitted via the unsolicited data mechanism (that is,
  FETCH should be viewed as instructing the server to include the
  desired data along with any other data it wishes to transmit to the
  client).  There are three major categories of data that may be
  fetched.

  The first category is data that is associated with a message as an
  entity in the mailbox.  There are now three such items of data: the
  "internal date", the "RFC 822 size", and the "flags".  The internal
  date is the date and time that the message was placed in the mailbox.
  The RFC 822 size is subject to deletion in the future; it is the size
  in bytes of the message, expressed as an RFC 822 text string.
  Current clients only use it as part of a status display line.  The
  flags are a list of status flags associated with the message (see
  below).  All the first category data can be fetched by using the
  macro-fetch word "FAST"; that is, "FAST" expands to "(FLAGS
  INTERNALDATE RFC822.SIZE)".

  The second category is that data that describes the composition and
  delivery information of a message; that is, information such as the
  message sender, recipient lists, message-ID, subject, etc.  This is
  the information that is stored in the message header in RFC 822
  format message and is traditionally called the "envelope".  [Note:
  this should not be confused with the SMTP (RFC 821) envelope, which
  is strictly limited to delivery information.]  IMAP2 defines a
  structured and unambiguous representation for the envelope that is
  particularly suited for Lisp-based parsers.  A client can use the
  envelope for operations such as replying and not worry about RFC 822
  at all.  Envelopes are discussed in more detail below.  The first two
  categories of data can be fetched together by using the macro-fetch
  word "ALL"; that is, "ALL" expands to "(FLAGS INTERNALDATE
  RFC822.SIZE ENVELOPE)".

  The third category is that data that is intended for direct human
  viewing.  The present RFC 822 based IMAP2 defines three such items:
  RFC822.HEADER, RFC822.TEXT, and RFC822 (the latter being the two
  former appended together in a single text string).  RFC822.HEADER is
  the "raw", unprocessed RFC 822 format header of the message.
  Fetching "RFC822" is equivalent to fetching the RFC 822
  representation of the message as stored on the mailbox without any
  filtering or processing.

  An intelligent client will "FETCH ALL" for some (or all) of the
  messages in the mailbox for use as a presentation menu, and when the
  user wishes to read a particular message will "FETCH RFC822.TEXT" to
  get the message body.  A more primitive client could, of course,
  simply "FETCH RFC822" a`la POP-type functionality.

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RFC 1176 IMAP2 August 1990

  The client can alter certain data (currently only the flags) by a
  STORE command.  As an example, a message is deleted from a mailbox by
  a STORE command that includes the \DELETED flag as a flag being set.

  Other client operations include copying a message to another mailbox
  (COPY command), permanently removing deleted messages (EXPUNGE
  command), checking for new messages (CHECK command), and searching
  for messages that match certain criteria (SEARCH command).

  The client terminates the session with the LOGOUT command.  The
  server returns a "BYE" followed by an "OK".

  A Typical Scenario

          Client                          Server
          ------                          ------
                                      {Wait for Connection}
      {Open Connection}        -->
                                  <-- * OK IMAP2 Server Ready
                                      {Wait for command}
      A001 LOGIN Fred Secret   -->
                                  <-- A001 OK User Fred logged in
                                      {Wait for command}
      A002 SELECT INBOX        -->
                                  <-- * FLAGS (Meeting Notice \Answered
                                               \Flagged \Deleted \Seen)
                                  <-- * 19 EXISTS
                                  <-- * 2 RECENT
                                  <-- A0002 OK Select complete
                                      {Wait for command}
      A003 FETCH 1:19 ALL      -->
                                  <-- * 1 Fetch (......)
                                          ...
                                  <-- * 18 Fetch (......)
                                  <-- * 19 Fetch (......)
                                  <-- A003 OK Fetch complete
                                      {Wait for command}
      A004 FETCH 8 RFC822.TEXT -->
                                  <-- * 8 Fetch (RFC822.TEXT {893}
                                          ...893 characters of text...
                                  <-- )
                                  <-- A004 OK Fetch complete
                                      {Wait for command}

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RFC 1176 IMAP2 August 1990

      A005 STORE 8 +Flags \Deleted -->
                                  <-- * 8 Store (Flags (\Deleted
                                                 \Seen))
                                  <-- A005 OK Store complete
                                      {Wait for command}
      A006 EXPUNGE             -->
                                  <-- * 19 EXISTS
                                  <-- * 8 EXPUNGE
                                  <-- * 18 EXISTS
                                  <-- A006 Expunge complete
                                      {Wait for command}
      A007 LOGOUT              -->
                                  <-- * BYE IMAP2 server quitting
                                  <-- A007 OK Logout complete
      {Close Connection}       --><-- {Close connection}
                                      {Go back to start}

Conventions

  The following terms are used in a meta-sense in the syntax
  specification below:

     An ASCII-STRING is a sequence of arbitrary ASCII characters.

     An ATOM is a sequence of ASCII characters delimited by SP or CRLF.

     A CHARACTER is any ASCII character except """", "{", CR, LF, "%",
     or "\".

     A CRLF is an ASCII carriage-return character followed immediately
     by an ASCII linefeed character.

     A NUMBER is a sequence of the ASCII characters that represent
     decimal numerals ("0" through "9"), delimited by SP, CRLF, ",", or
     ":".

     A SP is the ASCII space character.

     A TEXT_LINE is a human-readable sequence of ASCII characters up to
     but not including a terminating CRLF.

  A common field in the IMAP2 protocol is a STRING, which may be an
  ATOM, QUOTED-STRING (a sequence of CHARACTERs inside double-quotes),
  or a LITERAL.  A literal consists of an open brace ("{"), a number, a
  close brace ("}"), a CRLF, and then an ASCII-STRING of n characters,
  where n is the value of the number inside the brace.  In general, a
  string should be represented as an ATOM or QUOTED-STRING if at all
  possible.  The semantics for QUOTED-STRING or LITERAL are checked
  before those for ATOM; therefore an ATOM used in a STRING may only

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  contain CHARACTERs.  Literals are most often sent from the server to
  the client; in the rare case of a client to server literal there is a
  special consideration (see the "+ text" response below).

  Another important field is the SEQUENCE, which identifies a set of
  messages by consecutive numbers from 1 to n where n is the number of
  messages in the mailbox.  A sequence may consist of a single number,
  a pair of numbers delimited by colon (equivalent to all numbers
  between those two numbers), or a list of single numbers or number
  pairs.  For example, the sequence 2,4:7,9,12:15 is equivalent to
  2,4,5,6,7,9,12,13,14,15 and identifies all those messages.

Definitions of Commands and Responses

    Summary of Commands and Responses

      Commands                            ||      Responses
      --------                            ||      -------
      tag NOOP                            ||      tag OK text
      tag LOGIN user password             ||      tag NO text
      tag LOGOUT                          ||      tag BAD text
      tag SELECT mailbox                  ||      * number message_data
      tag BBOARD bulletin_board           ||      * FLAGS flag_list
      tag FIND MAILBOXES pattern          ||      * SEARCH sequence
      tag FIND BBOARDS pattern            ||      * BBOARD string
      tag CHECK                           ||      * MAILBOX string
      tag EXPUNGE                         ||      * BYE text
      tag COPY sequence mailbox           ||      * OK text
      tag FETCH sequence data             ||      * NO text
      tag STORE sequence data value       ||      * BAD text
      tag SEARCH search_program           ||      + text

Commands

  tag NOOP

     The NOOP command returns an OK to the client.  By itself, it does
     nothing, but certain things may happen as side effects.  For
     example, server implementations that implicitly check the mailbox
     for new mail may do so as a result of this command.  The primary
     use of this command is to for the client to see if the server is
     still alive (and notify the server that the client is still alive,
     for those servers that have inactivity autologout timers).

  tag LOGIN user password

     The LOGIN command identifies the user to the server and carries
     the password authenticating this user.  This information is used

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     by the server to control access to the mailboxes.

     EXAMPLE:  A001 LOGIN SMITH SESAME
     logs in as user SMITH with password SESAME.

  tag LOGOUT

     The LOGOUT command 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.

  tag SELECT mailbox

     The SELECT 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 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 SELECT commands are permitted in a session, in which case
     the previous mailbox is automatically deselected when a new SELECT
     is made.

     The default mailbox for the SELECT command is INBOX, which is a
     special name reserved to mean "the primary mailbox for this user
     on this server".  The format of other mailbox names is operating
     system dependent (as of this writing, it reflects the filename
     path of the mailbox file on the current servers).

     It is customary, although not required, for the text of an OK
     response to the SELECT command to begin with either "[READ-ONLY]"
     or "[READ-WRITE]" to show the mailbox's access status.

     EXAMPLE: A002 SELECT INBOX
     selects the default mailbox.

  tag BBOARD bulletin_board

     The BBOARD command is equivalent to SELECT, and returns the same
     output.  However, it differs from SELECT in that its argument is a
     shared mailbox (bulletin board) name instead of an ordinary
     mailbox.  The format of a bulletin name is implementation
     specific, although it is strongly encouraged to use something that
     resembles a name in a generic sense and not a file or mailbox name

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     on the particular system.  There is no requirement that a bulletin
     board name be a mailbox name or a file name (in particular, Unix
     netnews has a completely different namespace from mailbox or file
     names).

     Support for BBOARD is optional.

  tag FIND MAILBOXES pattern

     The FIND MAILBOXES command accepts as an argument a pattern
     (including wildcards) that specifies some set of mailbox names
     that are usable by the SELECT command.  The format of mailboxes is
     implementation dependent.  The special mailbox name INBOX is not
     included in the output.

     Two wildcard characters are defined; "*" specifies any number
     (including zero) characters may match at this position and "%"
     specifies a single character may match at this position.  For
     example, FOO*BAR will match FOOBAR, FOOD.ON.THE.BAR and FOO.BAR,
     whereas FOO%BAR will match only FOO.BAR.  "*" will match all
     mailboxes.

     The FIND MAILBOXES command will return some set of unsolicited
     MAILBOX replies that have as their value a single mailbox name.

     EXAMPLE:  A002 FIND MAILBOXES *
               * MAILBOX FOOBAR
               * MAILBOX GENERAL
               A002 FIND completed

     Although the use of explicit file or path names for mailboxes is
     discouraged by this standard, it may be unavoidable.  It is
     important that the value returned in the MAILBOX unsolicited reply
     be usable in the SELECT command without remembering any path
     specification that may have been used in the FIND MAILBOXES
     pattern.

     Support for FIND MAILBOXES is optional.  If a client's attempt
     returns BAD as a response then the client can make no assumptions
     about what mailboxes exist on the server other than INBOX.

  tag FIND BBOARDS pattern

     The FIND BBOARDS command accepts as an argument a pattern that
     specifies some set of bulletin board names that are usable by the
     BBOARD command.  Wildcards are permitted as in FIND MAILBOXES.

     The FIND BBOARDS command will return some set of unsolicited

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     BBOARD replies that have as their value a single bulletin board
     name.

     EXAMPLE:  A002 FIND BBOARDS *
               * BBOARD FOOBAR
               * BBOARD GENERAL
               A002 FIND completed

     Support for FIND BBOARDS is optional.  If a client's attempt
     returns BAD as a response then the client can make no assumptions
     about what bulletin boards exist on the server, or that they exist
     at all.

  tag CHECK

     The CHECK command forces a check for new messages and a rescan of
     the mailbox for internal change for those implementations that
     allow multiple simultaneous read/write access to the same mailbox.
     It is recommend that periodic implicit checks for new mail be done
     by servers as well.  The server should send unsolicited EXISTS and
     RECENT responses with the current status before returning an OK to
     the client.

  tag EXPUNGE

     The EXPUNGE command permanently removes all messages with the
     \DELETED flag set in its flags from the mailbox.  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.  To ensure mailbox integrity and server/client
     synchronization, it is recommended that the server do an implicit
     check before commencing the expunge and again when the expunge is
     completed.  Furthermore, if the server allows multiple
     simultaneous access to the same mail file the server must lock the
     mail file for exclusive access while an expunge is taking place.

     EXPUNGE is not allowed if the user does not have write access to
     this mailbox.

  tag COPY sequence mailbox

     The COPY command copies the specified message(s) to the specified
     destination mailbox.  If the destination mailbox does not exist,
     the server should create it.  Before returning an OK to the
     client, the server should return an unsolicited <n> COPY response

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RFC 1176 IMAP2 August 1990

     for each message copied.  A copy should set the \SEEN flag for all
     messages that were successfully copied (provided, of course, that
     the user has write access to this mailbox).

     EXAMPLE:  A003 COPY 2:4 MEETING
     copies messages 2, 3, and 4 to mailbox "MEETING".

     COPY is not allowed if the user does not have write access to the
     destination mailbox.

  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)

     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.
                     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

     INTERNALDATE    The date and time the message was written to
                     the mailbox.

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RFC 1176 IMAP2 August 1990

     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 "raw" RFC 822 format header of the message
                     as stored on the server.

     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 as with RFC822 above.

     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.

     Note: An attempt to FETCH already-transmitted data may have no
     result.  See the Implementation Discussion below.

  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.

     STORE is not allowed if the user does not have write access to
     this mailbox.

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RFC 1176 IMAP2 August 1990

     EXAMPLE:  A003 STORE 2:4 +FLAGS (\DELETED)
     marks messages 2, 3, and 4 for deletion.

  tag SEARCH search_criteria

     The SEARCH command searches the mailbox for messages 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 case-independent substring of that
     field.  The currently defined criteria are:

     ALL             All messages in the mailbox; the default
                     initial criterion for ANDing.

     ANSWERED        Messages with the \ANSWERED flag set.

     BCC string      Messages which contain the specified string
                     in the envelope's BCC field.

     BEFORE date     Messages whose internal date is earlier than
                     the specified date.

     BODY string     Messages which contain the specified string
                     in the body of the message.

     CC string       Messages which contain the specified string
                     in the envelope's CC field.

     DELETED         Messages with the \DELETED flag set.

     FLAGGED         Messages with the \FLAGGED flag set.

     FROM string     Messages which contain the specified string
                     in the envelope's FROM field.

     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".

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RFC 1176 IMAP2 August 1990

     OLD             Messages which do not have the \RECENT flag
                     set.

     ON date         Messages whose internal date is the same as
                     the specified date.

     RECENT          Messages which have the \RECENT flag set.

     SEEN            Messages which have the \SEEN flag set.

     SINCE date      Messages whose internal date is later than
                     the specified date.

     SUBJECT string  Messages which contain the specified string
                     in the envelope's SUBJECT field.

     TEXT string     Messages which contain the specified string.

     TO string       Messages which contain the specified string in
                     the envelope's TO field.

     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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RFC 1176 IMAP2 August 1990

Responses

  tag OK text

     This response identifies successful completion of the command with
     that tag.  The text is a line of human-readable text that may be
     useful in a protocol telemetry log for debugging purposes.

  tag NO 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.

  tag BAD 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.

  * number message_data

     This response occurs as a result of several different commands.
     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.

     STORE data
             Obsolete and functionally equivalent to FETCH.

     FETCH data
             This is the principle means by which data about a
             message is returned to the client.  The data is in a
             Lisp-like S-expression property list form.  The current
             properties are:

        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

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RFC 1176 IMAP2 August 1990

                     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

        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

        RFC822.SIZE   A number indicating the number of
                      characters in the message as expressed

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RFC 1176 IMAP2 August 1990

                      in RFC 822 format.

        RFC822.TEXT   A string expressing the text body of the
                      message, omitting the RFC 822 header.

  * FLAGS flag_list

     This response occurs as a result of a SELECT 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.

  * 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".

  * BBOARD string

     This response occurs as a result of a FIND BBOARDS command.  The
     string is a bulletin board name that matches the pattern in the
     command.

  * MAILBOX string

     This response occurs as a result of a FIND MAILBOXES command.  The
     string is a mailbox name that matches the pattern in the command.

  * 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.

  * OK text

     This response identifies normal operation on the server.  No
     special action by the client is called for, however, the text
     should be displayed to the user in some fashion.  This is
     currently only used by servers at startup as a greeting message to
     show they are ready to accept the first command.

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RFC 1176 IMAP2 August 1990

  * NO text

     This response identifies a warning from the server that does not
     affect the overall results of any particular request.  The text is
     a line of human-readable text that should be presented to the user
     as a warning of improper operation.

  * BAD text

     This response identifies a serious error at the server; it may
     also indicate faulty protocol 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.  It should also be recorded in any telemetry as part of a
     bug report to the maintainer of the client and server.

  + text

     This response identifies that the server is ready to accept the
     text of a literal from the client.  Normally, a command from the
     client is a single text line.  If the server detects an error in
     the command, it can simply discard the remainder of the line.  It
     cannot do this 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.

     In practice, this condition is rarely encountered.  In the current
     protocol, the only client command likely to contain a literal is
     the LOGIN command.  Consider a server that validates the user
     before checking the password.  If the password contains "funny"
     characters and hence is sent as a literal, then if the user is
     invalid an error would occur before the password is parsed.

     No such synchronization protection is provided for literals sent
     from the server to the client, for performance reasons.  Any
     synchronization problems in this direction would be caused by a
     bug in the client or server.

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RFC 1176 IMAP2 August 1990

Sample IMAP2 session

  The following is a transcript of an IMAP2 session.  Server output is
  identified by "S:" and client output by "U:".  In cases where lines
  are too long to fit within the boundaries of this document, the line
  is continued on the next line.

  S:   * OK SUMEX-AIM.Stanford.EDU Interim Mail Access Protocol II Service
        6.1(349) at Thu, 9 Jun 88 14:58:30 PDT
  U:   a001 login crispin secret
  S:   a002 OK User CRISPIN logged in at Thu, 9 Jun 88 14:58:42 PDT, job 76
  U:   a002 select inbox
  S:   * FLAGS (Bugs SF Party Skating Meeting Flames Request AI Question
        Note \XXXX \YYYY \Answered \Flagged \Deleted \Seen)
  S:   * 16 EXISTS
  S:   * 0 RECENT
  S:   a002 OK Select complete
  U:   a003 fetch 16 all
  S:   * 16 Fetch (Flags (\Seen) InternalDate " 9-Jun-88 12:55:44 PDT"
        RFC822.Size 637 Envelope ("Sat, 4 Jun 88 13:27:11 PDT"
        "INFO-MAC Mail Message" (("Larry Fagan" NIL "FAGAN"
        "SUMEX-AIM.Stanford.EDU")) (("Larry Fagan" NIL "FAGAN"
        "SUMEX-AIM.Stanford.EDU")) (("Larry Fagan" NIL "FAGAN"
        "SUMEX-AIM.Stanford.EDU")) ((NIL NIL "rindflEISCH"
        "SUMEX-AIM.Stanford.EDU")) NIL NIL NIL
        "<[email protected]>"))
  S:   a003 OK Fetch completed
  U:   a004 fetch 16 rfc822
  S:   * 16 Fetch (RFC822 {637}
  S:   Mail-From: RINDFLEISCH created at  9-Jun-88 12:55:43
  S:   Mail-From: FAGAN created at  4-Jun-88 13:27:12
  S:   Date: Sat, 4 Jun 88 13:27:11 PDT
  S:   From: Larry Fagan  <[email protected]>
  S:   To: [email protected]
  S:   Subject: INFO-MAC Mail Message
  S:   Message-ID: <[email protected]>
  S:   ReSent-Date: Thu, 9 Jun 88 12:55:43 PDT
  S:   ReSent-From: TC Rindfleisch <[email protected]>
  S:   ReSent-To: [email protected],
        [email protected]
  S:   ReSent-Message-ID:
        <[email protected]>
  S:
  S:   The file is <info-mac>usenetv4-55.arc  ...
  S:   Larry
  S:   -------
  S:   )
  S:   a004 OK Fetch completed

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RFC 1176 IMAP2 August 1990

  U:   a005 logout
  S:   * BYE DEC-20 IMAP II server terminating connection
  S:   a005 OK SUMEX-AIM.Stanford.EDU Interim Mail Access Protocol
        Service logout

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RFC 1176 IMAP2 August 1990

Implementation Discussion

  There are several advantages to the scheme of tags and unsolicited
  responses.  First, the infamous synchronization problems of SMTP and
  similar protocols do not happen with tagged commands; a command is
  not considered satisfied until a response with the same tag is seen.
  Tagging allows an arbitrary amount of other responses ("unsolicited"
  data) to be sent by the server with no possibility of the client
  losing synchronization.  Compare this with the problems that FTP or
  SMTP clients have with continuation, partial completion, and
  commentary reply codes.

  Another advantage is that a non-lockstep client implementation is
  possible.  The client could send a command, and entrust the handling
  of the server responses to a different process that would signal the
  client when the tagged response comes in.  Under certain
  circumstances, the client may have more than one command outstanding.

  It was observed that synchronization problems can occur with literals
  if the literal is not recognized as such.  Fortunately, the cases in
  which this can happen are rare; a mechanism (the special "+" tag
  response) was introduced to handle those few cases.  The proper way
  to address this problem is probably to move towards a record-oriented
  architecture instead of the text stream model provided by TCP.

  An IMAP2 client must maintain a local cache of data from the mailbox.
  This cache is an incomplete model of the mailbox, and at startup is
  empty.  A listener processes all unsolicited data, and updates the
  cache based on this data.  If a tagged response arrives, the listener
  unblocks the process that sent the tagged request.

  Unsolicited data needs some discussion.  Unlike most protocols, in
  which the server merely does the client's bidding, an IMAP2 server
  has a semi-autonomous role.  By sending "unsolicited data", the
  server is in effect sending a command to the client -- to update or
  extend the client's cache with new information from the server.  In
  other words, a "fetch" command is merely a request to the server to
  ensure that the client's cache has the most up-to-date version of the
  requested information.  A server acknowledgement to the "fetch" is a
  statement that all the requested data has been sent.

  Although no current server does this, a server is not obliged by the
  protocol to send data that it has already sent and is unchanged.  An
  exception to this is the actual message text fetching operations
  (RFC822, RFC822.HEADER, and RFC822.TEXT), owing to the possibly
  excessive resource consumption of maintaining this data in a cache.
  It can not be assumed that a FETCH will transmit any data; only that
  an OK to the FETCH means that the client's cache has the most up-to-

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RFC 1176 IMAP2 August 1990

  date information.

  When a mailbox is selected, the initial unsolicited data from the
  server arrives.  The first piece of data is the number of messages.
  By sending a new EXISTS unsolicited data message the server causes
  the client to resize its cache (this is how newly arrived mail is
  handled).  If the client attempts to access information from the
  cache, it will encounter empty spots that will trigger "fetch"
  requests.  The request would be sent, some unsolicited data including
  the answer to the fetch will flow back, and then the "fetch" response
  will unblock the client.

  People familiar with demand-paged virtual memory operating system
  design will recognize this model as being similar to page-fault
  handling on a demand-paged system.

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RFC 1176 IMAP2 August 1990

Formal Syntax

  The following syntax specification uses the augmented Backus-Naur
  Form (BNF) notation as specified in RFC 822 with one exception; the
  delimiter used with the "#" construct is a single space (SP) and not
  a comma.

  address         ::= "(" addr_name SP addr_adl SP addr_mailbox SP
                      addr_host ")"

  addr_adl        ::= nil / string

  addr_host       ::= nil / string

  addr_mailbox    ::= nil / string

  addr_name       ::= nil / string

  bboard          ::= "BBOARD" SP string

  check           ::= "CHECK"

  copy            ::= "COPY" SP sequence SP mailbox

  data            ::= ("FLAGS" SP flag_list / "SEARCH" SP 1#number /
                      "BYE" SP text_line / "OK" SP text_line /
                      "NO" SP text_line / "BAD" SP text_line)

  date            ::= string in form "dd-mmm-yy hh:mm:ss-zzz"

  envelope        ::= "(" env_date SP env_subject SP env_from SP
                      env_sender SP env_reply-to SP env_to SP
                      env_cc SP env_bcc SP env_in-reply-to SP
                      env_message-id ")"

  env_bcc         ::= nil / "(" 1*address ")"

  env_cc          ::= nil / "(" 1*address ")"

  env_date        ::= string

  env_from        ::= nil / "(" 1*address ")"

  env_in-reply-to ::= nil / string

  env_message-id  ::= nil / string

  env_reply-to    ::= nil / "(" 1*address ")"

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RFC 1176 IMAP2 August 1990

  env_sender      ::= nil / "(" 1*address ")"

  env_subject     ::= nil / string

  env_to          ::= nil / "(" 1*address ")"

  expunge         ::= "EXPUNGE"

  fetch           ::= "FETCH" SP sequence SP ("ALL" / "FAST" /
                      fetch_att / "(" 1#fetch_att ")")

  fetch_att       ::= "ENVELOPE" / "FLAGS" / "INTERNALDATE" /
                      "RFC822" / "RFC822.HEADER" / "RFC822.SIZE" /
                      "RFC822.TEXT"

  find            ::= "FIND" SP find_option SP string

  find_option     ::= "MAILBOXES" / "BBOARDS"

  flag_list       ::= ATOM / "(" 1#ATOM ")"

  literal         ::= "{" NUMBER "}" CRLF ASCII-STRING

  login           ::= "LOGIN" SP userid SP password

  logout          ::= "LOGOUT"

  mailbox         ::= "INBOX" / string

  msg_copy        ::= "COPY"

  msg_data        ::= (msg_exists / msg_recent / msg_expunge /
                      msg_fetch / msg_copy)

  msg_exists      ::= "EXISTS"

  msg_expunge     ::= "EXPUNGE"

  msg_fetch       ::= ("FETCH" / "STORE") SP "(" 1#("ENVELOPE" SP
                       envelope / "FLAGS" SP "(" 1#(recent_flag
                       flag_list) ")" / "INTERNALDATE" SP date /
                       "RFC822" SP string / "RFC822.HEADER" SP string /
                       "RFC822.SIZE" SP NUMBER / "RFC822.TEXT" SP
                       string) ")"

  msg_recent      ::= "RECENT"

  msg_num         ::= NUMBER

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RFC 1176 IMAP2 August 1990

  nil             ::= "NIL"

  noop            ::= "NOOP"

  password        ::= string

  recent_flag     ::= "\RECENT"

  ready           ::= "+" SP text_line

  request         ::= tag SP (noop / login / logout / select / check /
                      expunge / copy / fetch / store / search / find /
                      bboard) CRLF

  response        ::= tag SP ("OK" / "NO" / "BAD") SP text_line CRLF

  search          ::= "SEARCH" SP 1#("ALL" / "ANSWERED" /
                      "BCC" SP string / "BEFORE" SP string /
                      "BODY" SP string / "CC" SP string / "DELETED" /
                      "FLAGGED" / "KEYWORD" SP atom / "NEW" / "OLD" /
                      "ON" SP string / "RECENT" / "SEEN" /
                      "SINCE" SP string / "TEXT" SP string /
                      "TO" SP string / "UNANSWERED" / "UNDELETED" /
                      "UNFLAGGED" / "UNKEYWORD" / "UNSEEN")

  select          ::= "SELECT" SP mailbox

  sequence        ::= NUMBER / (NUMBER "," sequence) / (NUMBER ":"
                      sequence)

  store           ::= "STORE" SP sequence SP store_att

  store_att       ::= ("+FLAGS" SP flag_list / "-FLAGS" SP flag_list /
                      "FLAGS" SP flag_list)

  string          ::= atom / """" 1*character """" / literal

  system_flags    ::= "\ANSWERED" SP "\FLAGGED" SP "\DELETED" SP
                      "\SEEN"

  tag             ::= atom

  unsolicited     ::= "*" SP (msg_num SP msg_data / data) CRLF

  userid          ::= string

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RFC 1176 IMAP2 August 1990

Implementation Status

  This information is current as of this writing.

  The University of Washington has developed an electronic mail client
  library called the "C-Client". It provides complete IMAP2, SMTP, and
  local mailbox (both /usr/spool/mail and mail.txt formats) services in
  a well-defined way to a user interface main program.  Using the C-
  Client, the University of Washington has created an operational
  client for BSD Unix and two operational clients (one basic, one
  advanced) for the NeXT.

  Stanford University/SUMEX has developed operational IMAP2 clients for
  Xerox Lisp machines, Texas Instruments Explorers, and the Apple
  Macintosh.  The core of the Macintosh client is an early version of
  the C-Client.  SUMEX has also developed IMAP2 servers for TOPS-20 and
  BSD Unix.

  All of the above software is in production use, with enthusiastic
  local user communities.  Active development continues on the
  Macintosh and C-Client based clients and the BSD Unix server.  This
  software is freely available from the University of Washington and
  SUMEX.

  IMAP2 software exists for other platforms; for example Nippon
  Telephone and Telegraph (NTT) has developed an operational IMAP2
  client for the NTT ELIS.  Several organizations are working on a PC
  client.

  IMAP2 can be used to access mailboxes at very remote sites, where
  echo delays and frequent outages make TELNET and running a local mail
  reader intolerable.  For example, from a desktop workstation on the
  University of Washington local network the author routinely uses
  IMAP2 to read and manage mailboxes on various University of
  Washington local servers, at two systems at Stanford University, at a
  Milnet site, and at a site in Tokyo, Japan.

  This specification does not make any formal definition of size
  restrictions, but the DEC-20 server has the following limitations:

   . length of a mailbox: 7,077,888 characters
   . maximum number of messages: 18,432 messages
   . length of a command line: 10,000 characters
   . length of the local host name: 64 characters
   . length of a "short" argument: 39 characters
   . length of a "long" argument: 491,520 characters
   . maximum amount of data output in a single fetch:
     655,360 characters

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RFC 1176 IMAP2 August 1990

  To date, nobody has run up against any of these limitations, many of
  which are substantially larger than most current user mail reading
  programs.

Acknowledgements

  Bill Yeager and Rich Acuff both contributed invaluable suggestions in
  the evolution of IMAP2 from the original IMAP.  James Rice pointed
  out several ambiguities in the previous IMAP2 specification and
  otherwise would not allow me to leave bad enough along.  Laurence
  Lundblade reviewed a draft of this version and made several helpful
  suggestions.

  Many dedicated individuals have worked on IMAP2 software, including:
  Mark Crispin, Frank Gilmurray, Christopher Lane, Hiroshi Okuno,
  Christopher Schmidt, and Bill Yeager.

  Any mistakes, flaws, or sins of omission in this IMAP2 protocol
  specification are, however, strictly my own; and the mention of any
  name above does not imply an endorsement.

Security Considerations

  Security issues are not discussed in this memo.

Author's Address

  Mark R. Crispin
  Panda Programming
  6158 Lariat Loop NE
  Bainbridge Island, WA 98110-2020

  Phone: (206) 842-2385

  EMail: [email protected]

Crispin