Network Working Group                                         M. Crispin
Request for Comments: 1064                                     SUMEX-AIM
                                                               July 1988

INTERACTIVE MAIL ACCESS PROTOCOL - VERSION 2

Status of this Memo

   This RFC suggests a method for workstations to dynamically access
   mail from a mailbox server ("repository").  This RFC specifies a
   standard for the SUMEX-AIM community and a proposed experimental
   protocol for the Internet community.  Discussion and suggestions for
   improvement are requested.  Distribution of this memo is unlimited.

Introduction

   The intent of the Interactive Mail Access Protocol, Version 2 (IMAP2)
   is to allow a workstation or similar small machine to access
   electronic mail from a mailbox server.  IMAP2 is the protocol used by
   the SUMEX-AIM MM-D (MM Distributed) mail system.

   Although different in many ways from POP2 (RFC 937), IMAP2 may be
   thought of as a functional superset of POP2, and the POP2 RFC was
   used as a model for this RFC.  There was a cognizant reason for this;
   RFC 937 deals with an identical problem and it was desirable to offer
   a basis for comparison.

   Like POP2, 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).  A comparison with the DMSP protocol of
   PCMAIL can be found at the end of "System Model and Philosophy"
   section.

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

System Model and Philosophy

   Electronic mail is a primary means of communication for the widely
   spread SUMEX-AIM community.  The advent of distributed workstations
   is forcing a significant rethinking of the mechanisms employed to
   manage such mail.  With mainframes, each user tends to receive and
   process mail at the computer he used most of the time, his "primary
   host".  The first inclination of many users when an independent
   workstation is placed in front of them is to begin receiving mail at
   the workstation, and, in fact, many vendors have implemented

   facilities to do this.  However, this approach has several
   disadvantages:

      (1) Workstations (especially Lisp workstations) have a software
      design that gives full control of all aspects of the system to the
      user at the console.  As a result, background tasks, like
      receiving mail, could well be kept from running for long periods
      of time either because the user is asking to use all of the
      machine's resources, or because, in the course of working, the
      user has (perhaps accidentally) manipulated the environment in
      such a way as to prevent mail reception.  This could lead to
      repeated failed delivery attempts by outside agents.

      (2) The hardware failure of a single workstation could keep its
      user "off the air" for a considerable time, since repair of
      individual workstation units might be delayed.  Given the growing
      number of workstations spread throughout office environments,
      quick repair would not be assured, whereas a centralized mainframe
      is generally repaired very soon after failure.

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

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

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

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

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

   In defining a mail access protocol, it is important to keep in mind
   that the client and server form a macrosystem, in which it should be
   possible to exploit the strong points of both while compensating for
   each other's weaknesses.  Furthermore, it's desirable to allow for a
   growth path beyond the hoary text-only RFC 822 protocol.  Unlike
   POP2, IMAP2 has extensive features for remote searching and parsing
   of messages on the server.  For example, a free text search
   (optionally in conjunction with other searching) can be made
   throughout the entire mailbox by the server and the results made
   available to the client without the client having to transfer the
   entire mailbox and searching itself.  Since remote parsing of a
   message into a structured (and standard format) "envelope" is
   available, a client can display envelope information and implement
   commands such as REPLY without having any understanding of how to
   parse RFC 822, etc. headers.

   Additionally, IMAP2 offers several facilities for managing a mailbox
   beyond the simple "delete message" functionality of POP2.

   In spite of this, IMAP2 is a relatively simple protocol.  Although
   servers should implement the full set of IMAP2 functions, a simple
   client can be written which uses IMAP2 in much the way as a POP2
   client.

   IMAP2 differs from the DMSP protocol of PCMAIL (RFC 1056) in a more
   fundamental manner, reflecting the differing architectures of MM-D
   and PCMAIL.  PCMAIL is either an online ("interactive mode"), or

   offline ("batch mode") system.  MM-D 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 which summarize the
   message.  The server and client synchronize their states when the
   DMSP connection starts up, and, if a client has not accessed the
   server for a while, the client does a complete reset (reload) of its
   state from the server.

   In MM-D, the client/server relationship lasts only for the duration
   of the IMAP2 connection.  All mailbox state is maintained on the
   server.  There is no registration of clients.  The function of a
   descriptor is handled by a structured representation of the message
   "envelope".  This structure makes it unnecessary for a client to know
   anything about RFC 822 parsing.  There is no synchronization since
   the client does not remember state between 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 explicit support for bulletin boards which are only handled
   implicitly in IMAP2.  DMSP has functions for sending messages,
   printing messages, listing mailboxes, and changing passwords, all of
   which are done outside of IMAP2.  DMSP has 16 binary flags of which 8
   are defined by the system.  IMAP has flag names; there are currently
   5 defined system flag names and a facility for some number (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 relatively small set of "interesting" messages, which
   were either "new" mail or mail based upon 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
   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.  When commands are received the
   server acts on them and responds with responses, often interspersed
   with data.

   The client opens a connection, waits for the greeting, then sends a
   LOGIN command with user name and password arguments to establish
   authorization.  Following an OK response from the server, the client
   then sends a SELECT command to access the desired mailbox.  The
   user's default mailbox has a special reserved name of "INBOX" which
   is independent of the operating system that the server is implemented
   on.  The server will generally send a list of valid flags, number of
   messages, and number of messages arrived since last access for this
   mailbox as 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 by means of FETCH commands.  The
   actual data is transmitted via the unsolicited data mechanism (that
   is, FETCH should be viewed as poking the server to include the
   desired data along with any other data it wishes to transmit to the
   client).  There are three major categories of data which may be
   fetched.

   The first category is that data which is associated with a message as
   an entity in the mailbox.  There are presently three such items of
   data: the "internal date", the "RFC 822 size", and the "flags".  The
   internal date is the date and time that the message was placed in the
   mailbox.  The RFC 822 size is subject to deletion in the future; it
   is the size in bytes of the message, expressed as an RFC 822 text
   string.  Current clients only use it as part of a status display

   line.  The flags are a list of status flags associated with the
   message (see below).  All of the first category data can be fetched
   by using the macro-fetch word "FAST"; that is, "FAST" expands to
   "(FLAGS INTERNALDATE RFC822.SIZE)".

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

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

   Typically, a client will "FETCH ALL" for some or all of the messages
   in the mailbox for use as a presentation menu, and when the user
   wishes to read a particular message will "FETCH RFC822.TEXT" to get
   the message body.  A more primitive client could, of course, simply
   "FETCH RFC822" a la POP2-type functionality.

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

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

   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}

       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 which represent
      decimal numerals ("0" through "9"), delimited by SP, CRLF, ",", or
      ":".

      A SP is the ASCII space character.

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

   One of the most common fields in the 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 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 indicating all numbers between
   those two numbers, or a list of single numbers and/or number pairs.
   For example, the sequence 2,4:7,9,12:15 is equivalent to
   2,4,5,6,7,9,12,13,14,15 and identifies all of those messages.

Definitions of Commands and Responses

     Summary of Commands and Responses

       Commands                            ||      Responses
       --------                            ||      -------
       tag NOOP                            ||      tag OK text
       tag LOGIN user password             ||      tag NO text
       tag LOGOUT                          ||      tag BAD text
       tag SELECT mailbox                  ||      * message_number data
       tag CHECK                           ||      * FLAGS flag_list
       tag EXPUNGE                         ||      * SEARCH sequence
       tag COPY sequence mailbox           ||      * BYE text
       tag FETCH sequence data             ||      * OK text
       tag STORE sequence data value       ||      * NO text
       tag SEARCH search_program           ||      * BAD text
                                           ||      + text

Commands

   tag NOOP

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

   tag LOGIN user password

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

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

   tag LOGOUT

      The LOGOUT command indicates the client is done with the session.
      The server sends an unsolicited BYE response before the (tagged)
      OK response, and then closes the connection.

   tag SELECT mailbox

      The SELECT command selects a particular mailbox.  The server must

      check that the user is permitted read access to this mailbox.
      Prior to returning an OK to the client, the server must send an
      unsolicited FLAGS and <n> EXISTS response to the client giving the
      flags list for this mailbox (simply the system flags if this
      mailbox doesn't have any special flags) and the number of messages
      in the mailbox.  It is also recommended that the server send a <n>
      RECENT unsolicited response to the client for the benefit of
      clients which make use of the number of new messages in a mailbox.

      Multiple SELECT commands are permitted in a session, in which case
      the prior mailbox is deselected first.

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

      EXAMPLE: A002 SELECT INBOX selects the default mailbox.

   tag CHECK

      The CHECK command forces a check for new messages and a rescan of
      the mailbox for internal change for those implementations which
      allow multiple simultaneous read/write access to the same mailbox
      (e.g., TOPS-20).  It is recommend that periodic implicit checks
      for new mail be done by servers as well.  The server should send
      an unsolicited <n> EXISTS response prior to returning an OK to the
      client.

   tag EXPUNGE

      The EXPUNGE command permanently removes all messages with the
      \DELETED flag set in its flags from the mailbox.  Prior to
      returning an OK to the client, for each message which is removed,
      an unsolicited <n> EXPUNGE response is sent indicating which
      message was removed.  The message number of each subsequent
      message in the mailbox is immediately decremented by 1; this means
      that if the last 5 messages in a 9-message mail file are expunged
      you will receive 5 "* 5 EXPUNGE" responses.  To ensure mailbox
      integrity and server/client synchronization, it is recommended
      that the server do an implicit check prior to commencing the
      expunge and again when the expunge is completed.  Furthermore, if
      the server allows multiple simultaneous access to the same 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.  Prior to returning an OK to the
      client, the server should return an unsolicited <n> COPY response
      for each message copied.  A copy should set the \SEEN flag for all
      messages which 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 which are set for this message.
                      This may include the following system flags:

                              \RECENT    Message arrived since
                                          last read of this mail
                                          file
                              \SEEN      Message has been read
                              \ANSWERED  Message has been answered
                              \FLAGGED   Message is "flagged" for
                                          urgent/special attention
                              \DELETED   Message is "deleted" for
                                          removal by later EXPUNGE

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

      RFC822          The message in RFC 822 format.

      RFC822.HEADER   The RFC 822 format header of the message.

      RFC822.SIZE     The number of characters in the message as
                      expressed in RFC 822 format.

      RFC822.TEXT     The text body of the message, omitting the
                      RFC 822 header.

      EXAMPLES:

      A003 FETCH 2:4 ALL
         fetches the flags, internal date, RFC 822 size, and envelope
         for messages 2, 3, and 4.

      A004 FETCH 3 RFC822
         fetches the RFC 822 representation for message 3.

      A005 FETCH 4 (FLAGS RFC822.HEADER)
         fetches the flags and RFC 822 format header for message 4.

   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.

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

   tag SEARCH search_criteria

      The SEARCH command searches the mailbox for messages which match
      the given set of criteria.  The unsolicited SEARCH <1#number>
      response from the server is a list of messages which express the
      intersection (AND function) of all the messages.  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.

      KEYWORD flag    Messages with the specified flag set.

      NEW             Messages which have the \RECENT flag set but
                      not the \SEEN flag.  This is functionally
                      equivalent to "RECENT UNSEEN".

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

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

      RECENT          Messages which have the \RECENT flag set.

      SEEN            Messages which have the \SEEN flag set.

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

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

      TEXT string     Messages which contain the specified string.

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

      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.

         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.

Responses

   tag OK text

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

   tag NO text

      This response identifies unsuccessful completion of the command
      with the indicated tag.  The text is a line of human-readable text
      which probably should be displayed to the user in an error report
      by the client.

   tag BAD text

      This response indicates faulty protocol received from the client
      and indicates a bug in the client.  The text is a line of human-
      readable text which should be recorded in any telemetry as part of
      a bug report to the maintainer of the client.

   * 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
              last 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
              Functionally equivalent to FETCH, only it happens as a
              result of a STORE command.

      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 which describes the

                      envelope of a message.  The envelope is computed
                      by the server by parsing the RFC 822 header into
                      the component parts, defaulting various fields
                      as necessary.

                      The fields of the envelope are in the following
                      order: date, subject, from, sender, reply-to, to,
                      cc, bcc, in-reply-to, and message-id.  The date,
                      subject, in-reply-to, and message-id fields are
                      strings.  The from, sender, reply-to, to, cc,
                      and bcc fields are lists of addresses.

                      An address is an S-expression format list which
                      describes an electronic mail address.  The fields
                      of an address are in the following order:
                      personal name, source-route (a.k.a. the
                      at-domain-list in SMTP), mailbox name, and
                      host name.

                      Any field of an envelope or address which is
                      not applicable is presented as the atom NIL.
                      Note that the server must default the reply-to
                      and sender fields from the from field; a client is
                      not expected to know to do this.

         FLAGS        An S-expression format list of flags which are set
                      for this message.  This may include the following
                      system flags:

                      \RECENT       Message arrived since last
                                     read of this mail file
                      \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
                       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) which 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 those messages which match the search criteria.
      Each number is delimited by a space, e.g., "SEARCH 2 3 6".

   * BYE text

      This response indicates that the server is about to close the
      connection.  The text is a line of human-readable text which
      should be displayed to the user in a status report by the client.
      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 indicates that the server is alive.  No special
      action on the part of the client is called for.  This is presently
      only used by servers at startup as a greeting message indicating
      that they are ready to accept the first command.  The text is a
      line of human-readable text which may be logged in protocol
      telemetry.

   * NO text

      This response indicates some operational error at the server which
      cannot be traced to any protocol command.  The text is a line of
      human-readable text which should be logged in protocol telemetry
      for the maintainer of the server and/or the client.  No known
      server currently outputs such a response.

   * BAD text

      This response indicates some protocol error at the server which

      cannot be traced to any protocol command.  The text is a line of
      human-readable text which should be logged in protocol telemetry
      for the maintainer of the server and/or the client.  This
      generally indicates a protocol synchronization problem on the part
      of the client, and examination of the protocol telemetry is
      advised to determine the cause of the problem.

   + text

      This response indicates that the server is ready to accept the
      text of a literal from the client.  Normally, a command from the
      client is a single text line.  If the server detects an error in
      the command, it can simply discard the remainder of the line.  It
      cannot do this in the case of commands which contain literals,
      since a literal can be an arbitrarily long amount of text, and the
      server may not even be expecting a literal.  This mechanism is
      provided so the client knows not to send a literal until the
      server definitely expects it, preserving client/server
      synchronization.

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

      No such synchronization protection is provided for literals sent
      from the server to the client, for performance reasons.  Any
      synchronization problems in this direction would be due to a bug
      in the client or server and not for some operational problem.

Sample IMAP2 session

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

   S:      * OK SUMEX-AIM.Stanford.EDU 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:
               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
               "<12403828905.13.FAGAN@SUMEX-AIM.Stanford.EDU>"))
   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  <FAGAN@SUMEX-AIM.Stanford.EDU>
   S:   To: rindflEISCH@SUMEX-AIM.Stanford.EDU
   S:   Subject: INFO-MAC Mail Message
   S:   Message-ID: <12403828905.13.FAGAN@SUMEX-AIM.Stanford.EDU>
   S:   ReSent-Date: Thu, 9 Jun 88 12:55:43 PDT
   S:   ReSent-From: TC Rindfleisch <Rindfleisch@SUMEX-AIM.Stanford.EDU>
   S:   ReSent-To: Yeager@SUMEX-AIM.Stanford.EDU,
                   Crispin@SUMEX-AIM.Stanford.EDU
   S:   ReSent-Message-ID:
           <12405133897.80.RINDFLEISCH@SUMEX-AIM.Stanford.EDU>
   S:
   S:   The file is <info-mac>usenetv4-55.arc  ...
   S:   Larry
   S:   -------

   S:   )
   S:   pa004 OK Fetch completed
   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

Implementation Discussion

   As of this writing, SUMEX has completed an IMAP2 client for Xerox
   Lisp machines written in hybrid Interlisp/CommonLisp and is beta-
   testing a client for TI Explorers written entirely in CommonLisp.
   SUMEX has also completed a portable IMAP2 client protocol library
   module written in C.  This library, with the addition of a small main
   program (primarily user interface) and a TCP/IP driver, became a
   rudimentary remote system mail-reading program under Unix.  The first
   production use of this library will be as a part of a MacII client
   which is under development.

   As of this writing, SUMEX has completed IMAP2 servers for TOPS-20
   written in DEC-20 assembly language and 4.2/3 BSD Unix written in C.
   The TOPS-20 server is fully compatible with MM-20, the standard
   TOPS-20 mailsystem, and requires no special action or setup on the
   part of the user.  The INBOX under TOPS-20 is the user's MAIL.TXT.
   The TOPS-20 server also supports multiple simultaneous access to the
   same mailbox, including simultaneous access between the IMAP2 server
   and MM-20.  The 4.2/3 BSD Unix server requires that the user use
   mail.txt format which is compatible only with SRI MM-32 or Columbia
   MM-C.  The 4.2/3 BSD Unix server only allows simultaneous read
   access; write access must be exclusive.

   The Xerox Lisp client and DEC-20 server have been in production use
   for over a year; the Unix server was put into production use a few
   months ago.  IMAP2 has been used to access mailboxes at remote sites
   from a local workstation via the Internet.  For example, from the
   Stanford local network the author has read his mailbox at a Milnet
   site.

   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

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

   There are several advantages to the scheme of tags and 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 which would signal the
   client when the tagged response comes in.  Under certain
   circumstances, the client could even 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 relatively rare; a mechanism (the special
   "+" tag response) was introduced to handle those few cases which
   could happen.  The proper way to address this problem in all cases is
   probably to move towards a record-oriented architecture instead of
   the text stream model provided by TCP.

   Unsolicited data needs some discussion.  Unlike most protocols, in
   which the server merely does the client's bidding, an IMAP2 server
   has a semi-autonomous role.  By means of sending "unsolicited data",
   the server is in effect sending a command to the client -- to update
   and/or extend its (incomplete) model of the mailbox with new
   information from the server.  In this viewpoint, a "fetch" command is
   merely a request to the server to include the desired data in any
   other "unsolicited" data the server may send, and a server
   acknowledgement to the "fetch" is a statement that all the requested
   data has been sent.

   In terms of implementation, the client may have a local cache of data
   from the mailbox.  This cache is incomplete, 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 which sent the tagged request.

   Perhaps as a result of opening a mailbox, unsolicited data from the
   server arrives.  The first piece of data is the number of messages.
   This is used to size the cache; note that by sending a new "number of
   messages" unsolicited data message the cache would be re-sized (this
   is how newly arrived mail is handled).  If the client attempts to
   access information from the cache, it will encounter empty spots
   which will trigger "fetch" requests.  The request would be sent, some

   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 very similar to page-fault
   handling on a demand-paged system.

Formal Syntax

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

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

   addr_adl        ::= nil / string

   addr_host       ::= nil / string

   addr_mailbox    ::= nil / string

   addr_name       ::= nil / 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 ")"

   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"

   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

   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) 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

Acknowledgements

   Bill Yeager and Rich Acuff both contributed invaluable suggestions in
   the evolution of IMAP2 from the original IMAP.  The SUMEX IMAP2
   software was written by Mark Crispin (DEC-20 server, Xerox Lisp
   client, C client), Frank Gilmurray (Common Lisp client), Christopher
   Lane (Xerox Lisp client), and Bill Yeager (Unix server).  Any
   mistakes or flaws in this IMAP2 protocol specification are, however,
   strictly my own.