Network Working Group                                           J. Rice
Request for Comments: 1203                                     Stanford
Obsoletes: RFC 1064                                       February 1991


              INTERACTIVE MAIL ACCESS PROTOCOL - VERSION 3

Status of this Memo

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

Scope

   The following document is a modified version of RFC 1064, the
   definition of the IMAP2 protocol.  This RFC has been written
   specifically as a counter proposal to RFC 1176, which itself proposes
   modifications to IMAP2.  Sadly, RFC 1176 was made without internal
   consultation with the IMAP community, so we are in a position of
   feeling we have to present a counter proposal to what, if we do not
   act, will become a de facto standard.  The reasons for this counter
   proposal are numerous but fall mostly into the following categories:

      - IMAP2 is insufficiently powerful for a number of server/client
        interactions which we believe to be important.  RFC 1176
        negligibly enhances the functionality of IMAP2.

      - IMAP2 makes what we believe to be an erroneous definition for
        unsolicited vs. solicited data.  IMAP3 as specified herein
        attempts to correct this.  RFC 1176 makes no effort to remedy
        these problems.

      - RFC 1176 has explicitly modified the intent of RFC 1064 by
        allowing the server to make assumptions about the client's
        caching architecture.  We believe this to be a grave error
        and do not support it in this proposal.

      - RFC 1176 specifies a number of "optional" features in the
        protocol without specifying a suitable metaprotocol by which
        servers and clients can adequately negotiate over the set of
        implemented features.  This proposal specifies a mechanism
        by which servers and clients can come to an unambiguous
        understanding about which features are usable by each party.



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RFC 1203                         IMAP3                     February 1991



      - RFC 1176 pays only lip-service to being network protocol
        independent and, in fact assumes the use of TCP/IP.  Neither
        RFC 1064 nor this proposal make any such assumption.

   Although there are numerous other detailed objections to RFC 1176, we
   believe that the above will serve to show that we believe strongly in
   the importance of mailbox abstraction level mail protocols and, after
   a couple of years of use of IMAP2 under RFC 1064 we believe that we
   have a good enough understanding of the issues involved to be able to
   take the next step.

   It is important to take this next step because of the rapid pace of
   both mail system and user interface development.  We believe that,
   for IMAP not to die in its infancy, IMAP must be ready to respond to
   emerging ISO and RFC standards in mail, such as for multi-media mail.
   We believe that RFC 1176 not only provides a very small increment in
   functionality over RFC 1064 but also adds a number of bugs, which
   would be detrimental to the IMAP cause.  Thus we propose the
   following definition for IMAP3.

Compatibility notes:

   In revising the IMAP2 protocol it has been our intent, wherever
   possible to make upwards compatible changes to produce IMAP3.  There
   were, however, some places that had to be changed incompatibly in
   order to compensate for either ambiguities in the IMAP2 protocol as
   defined by RFC 1064 or behavior that proved undesirable in the light
   of experience.

   It is our goal, however, that existing IMAP2 clients should still be
   supported and that, at least for the foreseeable future, all IMAP3
   servers will support IMAP2 behavior as their default mode.

   The following are the major differences between this proposal, RFC
   1176 and RFC 1064:

      - In this proposal we specify a difference between "solicited" and
        "unsolicited" data sent from the server.  It is generally the
        case that data sent by the server can be sent either in response
        to an explicit request by the client or by the server of its own
        volition.  Any data that the server is required to sent to the
        client as the result of a request is said to be solicited and
        carries the same tag as the request that provoked it.  Any data
        sent by the server to the client that is not required by the
        protocol is said to be unsolicited and carries the special "*"
        tag.  RFC 1176 preserves the original RFC 1064 terminology that
        calls all such data sent by the server "unsolicited" even when



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        it is, in fact, solicited.

      - This proposal introduces the experimental concept of
        distinguishing between Generic, Canonical and Concrete keys,
        allowing the mailbox to be viewed as a relational database
        indexed by these keys.  This should allow the IMAP protocol
        to evolve away from its current reliance on RFC 822.  RFC 1176
        does not have such a unifying model.

      - The SEARCH command has been changed so as to allow multiple
        simultaneous searches to be made and to allow unsolicited
        search messages to be sent by the server.  Such a change is
        essential to allow more sophisticated servers that can process
        commands asynchronously, possibly substantially delaying
        searches over slow backing storage media, for example.  It is
        also important to allow servers to be able to send unsolicited
        search messages that might inform the client of interesting
        patterns of messages, such as new and unseen mail.

      - This proposal introduces a specific protocol for the negotiation
        of protocol versions and server features.  This is important
        because it allows client/server pairs to come to an agreement on
        what behavior is really available to it.  RFC 1176 introduces a
        number of "optional" commands, which are in some way analogous
        to "feature-introduced" commands in this proposal.  The principle
        distinction between these is that in RFC 1176 there is no way
        for a client to discover the set of optional commands, nor is
        there a way for it to determine whether a specific command
        really is supported, since RFC 1176 requires the use of the
        "BAD" response if a feature is not supported.  There is,
        therefore, no way for the client to determine why the attempted
        command did not work.  This also means that, for example, a
        client cannot disable certain user commands or make them
        invisible on menus if they are not supported, since there
        is no way for the client to discover whether the commands are
        indeed supported without trying to execute such a command.

      - This proposal introduces a mechanism for clients to create and
        delete user flags (keywords).  This is nor supported in either
        RFC 1176 or RFC 1064, requiring the user to add keys manually
        on the server, generally by editing some form of "init" file.

      - RFC 1064 has no mechanism for determining whether a mailbox is
        readonly or not.  RFC 1176 introduces a non-enforced convention
        of encoding data about the readonly status of a mailbox in the
        SELECT message's OK respose comment field.  This is not regular
        with respect to the rest of the protocol, in which the comment
        field is used for no purpose other than documentation.  This



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        proposal introduces specific protocol additions for the dynamic
        determination and modification of the readonly/readwrite status
        of mailboxes.

Introduction

   The intent of the Interactive Mail Access Protocol, Version 3 (IMAP3)
   is to allow a (possibly unreliable) workstation or similar machine to
   access electronic mail from a reliable mailbox server in an efficient
   manner.

   Although different in many ways from POP2 (RFC 937), IMAP3 may be
   thought of as a functional superset of POP2, and the POP2 RFC was
   used as a model for this RFC.  There was a cognizant reason for this;
   RFC 937 deals with an identical problem and it was desirable to offer
   a basis for comparison.

   Like POP2, IMAP3 specifies a means of accessing stored mail and not
   of posting mail; this function is handled by a mail transfer protocol
   such as SMTP (RFC 821).  A comparison with the DMSP protocol of
   PCMAIL can be found at the end of "System Model and Philosophy"
   section.

   This protocol assumes a reliable data stream such as provided by TCP
   or any similar protocol.  When TCP is used, the IMAP server listens
   on port 220.  When CHAOS is used the IMAP server listens for the
   logical contact name "IMAP3".

   Communication in IMAP is defined to be using the ASCII character
   interpretation of data.  Communication using other conventions may be
   possible by the selection of features on some servers.

System Model and Philosophy

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

      (1)  Workstations (especially Lisp workstations) have a software
           design that gives full control of all aspects of the system
           to the user at the console.  As a result, background tasks,



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           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,



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

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

   In defining a mail access protocol, it is important to keep in mind
   that the client and server form a macrosystem, in which it should be
   possible to exploit the strong points of both while compensating for
   each other's weaknesses.  Furthermore, it's desirable to allow for a
   growth path beyond the hoary text-only RFC 822 protocol.  Unlike
   POP2, IMAP3 has extensive features for remote searching and parsing
   of messages on the server.  For example, a free text search
   (optionally in conjunction with other searching) can be made
   throughout the entire mailbox by the server and the results made
   available to the client without the client having to transfer the
   entire mailbox and searching itself.  Since remote parsing of a
   message into a structured (and standard format) "envelope" is
   available, a client can display envelope information and implement
   commands such as REPLY without having any understanding of how to
   parse RFC 822, etc., headers.

   Additionally, IMAP3 offers several facilities for managing a mailbox
   beyond the simple "delete message" functionality of POP2.

   In spite of this, IMAP3 is a relatively simple protocol.  Although
   servers should implement the full set of IMAP3 functions, a simple
   client can be written which uses IMAP3 in much the way as a POP2
   client.

   IMAP3 differs from the DMSP protocol of PCMAIL (RFC 1056) in a more
   fundamental manner, reflecting the differing architectures of IMAP
   and PCMAIL.  PCMAIL is either an online ("interactive mode"), or
   offline ("batch mode") system.  IMAP is primarily an online system in
   which real-time and simultaneous mail access were considered



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   important.

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

   In IMAP, the client/server relationship lasts only for the duration
   of the IMAP3 connection.  All mailbox state is maintained on the
   server.  There is no registration of clients.  The function of a
   descriptor is handled by a structured representation of the message
   "envelope".  This structure makes it unnecessary for a client to know
   anything about RFC 822 parsing.  There is no synchronization since
   the client does not remember state between IMAP3 connections.  This
   is not a problem since in general the client never needs the entire
   state of the mailbox in a single session, therefore there isn't much
   overhead in fetching the state information that is needed as it is
   needed.

   There are also some functional differences between IMAP3 and DMSP.
   DMSP has functions for sending messages, printing messages, and
   changing passwords, all of which are done outside of IMAP3.  DMSP has
   16 binary flags of which 8 are defined by the system.  IMAP has flag
   names; there are currently 5 defined system flag names and a facility
   for some number (29 in the current implementations) of user flag
   names.  IMAP3 has a sophisticated message search facility in the
   server to identify interesting messages based on dates, addresses,
   flag status, or textual contents without compelling the client to
   fetch this data for every message.

   It was felt that maintaining state on the client is advantageous only
   in those cases where the client is only used by a single user, or if
   there is some means on the client to restrict access to another
   user's data.  It can be a serious disadvantage in an environment in
   which multiple users routinely use the same client, the same user
   routinely uses different clients, and where there are no access
   restrictions on the client.  It was also observed that most user mail
   access is to a relatively small set of "interesting" messages, which
   were either "new" mail or mail based upon some user-selected
   criteria. Consequently, IMAP3 was designed to easily identify those
   "interesting" messages so that the client could fetch the state of
   those messages and not those that were not "interesting".

   One crucial philosophical difference between IMAP and other common



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   mail protocols is that IMAP is a mailbox access protocol, not a
   protocol for manipulating mail files.  In the IMAP model, unlike
   other mail system models in which mail is stored in a linear mail
   file, no specification is made for the implementation architecture
   for mail storage.  Servers may choose to implement mailboxes as files
   but this is a detail of which the client can be totally unaware.

   What is more, in the IMAP model, mailboxes are viewed as mappings
   from keys into values.  There are broadly three types of keys,
   generic, canonical and concrete.  Generic keys are generic, mail
   protocol independent keys defined by IMAP which are meaningful across
   multiple mail encoding formats.  An example of such a generic key
   might be "TO", which would be associated with the "To:" field of an
   RFC 822 format message.

   Canonical keys represent the way in which the server can associate
   values that are generally "about" a certain key concept, possibly
   integrating several mail format specific fields, without having to
   worry the client with the particular details of any particular
   message format.  Thus, the canonical TO key (called $TO) could denote
   anything that could reasonably be construed as being directed towards
   someone.  Hence, in an RFC 822 message the server could find the
   union of the "To:", "Resent-To", "Apparently-To:" and "CC:" fields to
   be the appropriate value associated with the canonical $TO key.

   Concrete keys allow the client to gain access to certain mail format
   specific concepts, that are not pre-specified by the IMAP protocol,
   in a well defined manner.  For example, If the client asks for the
   value associated with the "APPARENTLY-TO" key then, if the message
   were to be in RFC 822 format, the server would look for a header
   field called "Apparently-To:".  If no such field is found or the
   field is not implemented or meaningful for the particular message
   format then the server will respond with the null value, called NIL,
   indicating the non-existence of the field.

   Thus, IMAP servers are at liberty to implement mailboxes as a
   relational databases if it seems convenient.  Indeed, we anticipate
   that future mail systems will tend to use database technology for the
   storage and indexing of mailboxes as a result of the pressure caused
   by the increasing size of mailboxes.

   Although for historical reasons IMAP is currently somewhat closely
   associated with RFC 822, we anticipate that future developments in
   IMAP will remove these mail format specific components and will move
   towards the generic model mentioned above.  This will allow IMAP more
   easily to incorporate such things as multi-media mail.





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The Protocol

   The IMAP3 protocol consists of a sequence of client commands and
   server responses to those commands, with extra information from the
   server data being sent asynchronously to and independent to the
   responses to client commands.  Unlike most Internet protocols,
   commands and responses are tagged.  That is, a command begins with a
   unique identifier (typically a short alphanumeric sequence such as a
   Lisp "gensym" function would generate e.g., A0001, A0002, etc.),
   called a tag.  The response to this command is given the same tag
   from the server.

   We distinguish between data sent by the server as the result of a
   client request, which we term "SOLICITED" and data sent by the server
   not as the result of a client request, which we term "UNSOLICITED".
   The server may send unsolicited data at any time that would not
   fragment another piece of data on the same stream rendering it
   unintelligible.  The server is contractually required, however, to
   return all data that is solicited by the client before the return of
   the completion signal for that command, i.e., all solicited data must
   be returned within the temporal extent of the request/completion
   acknowledgement wrapper.  This does not, however, preclude the
   simultaneous processing of multiple requests by the client, it simply
   requires that the client be confident that it has all the requested
   data when a request finishes.  This allows the implementation of both
   synchronous and asynchronous clients.

   Solicited data is identified by the tag of the initial request by the
   client.  Unsolicited data is identified by the special reserved tag
   of "*".  There is another special reserved tag, "+", discussed below.

   Note: the tagging of SOLICITED data is only permitted for a selected
   server version other than 2.0.

   No assumptions concerning serial or monolithic processing by the
   server can be made by a correct client.  The server is at liberty to
   process multiple requests by the same client in any order.  This
   allows servers to process costly searches over mailboxes on slow
   backing storage media in the background, while still preserving
   interactive performance.  Clients can, however, assume the
   serialization of the request/data/completion behavior mentioned
   above.

   When a connection is opened the server sends an unsolicited OK
   response as a greeting message and then waits for commands.  When
   commands are received the server acts on them and responds with
   responses, often interspersed with data.




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   The client opens a connection, waits for the greeting, then sends a
   LOGIN command with user name and password arguments to establish
   authorization.  Following an OK response from the server, the client
   then sends a SELECT command to access the desired mailbox.  The
   user's default mailbox has a special reserved name of "INBOX" which
   is independent of the operating system that the server is implemented
   on.  The server will generally send a list of valid flags, number of
   messages, and number of messages arrived since last access for this
   mailbox as solicited data, followed by an OK response.  The client
   may terminate access to this mailbox and access a different one with
   another SELECT command.

   Because the SELECT command affects the state of the server in a
   fundamental way, the server is required to process all outstanding
   commands for any given mailbox before sending the OK tag for the
   SELECT command.  Thus, the client will always know that all responses
   before an OK SELECT response will refer to the old mailbox and all
   responses following it will apply to the new mailbox.

   Because, in the real world, local needs or experimental work will
   dictate that servers will support both supersets of the defined
   behavior and incompatible changes, servers will support a
   SELECT.VERSION command and a SELECT.FEATURES command, the purpose of
   which is to allow clients to select the overall behavior and specific
   features that they want from a server.  The default behavior of any
   server is to process commands and to have interaction syntax the same
   as is specified by IMAP2 in RFC 1064.  A server may not behave in any
   other manner unless the SELECT.VERSION or SELECT.FEATURES commands
   are used to select different behavior.

   Over time, when groups of generally useful changes to the current,
   default behavior of the server are found, these will be collected
   together and incorporated in such a way that all of the features can
   be selected simply by selecting a particular major version number of
   the protocol.  It should be noted that the version numbers (both
   major and minor) selected by the SELECT.VERSION command denote
   versions of the IMAP protocol, not versions of the server per se.
   Thus, although in general changes to the protocol specification will
   be made in such a way that they are upwards compatible, this cannot
   be guaranteed.  No client should rely on tests of the form "if
   major_version > 2 then..." being valid for all protocol versions,
   since incompatible changes might be made in the future.

   The client reads mailbox information by means of FETCH commands.  The
   actual data is transmitted via the solicited data mechanism (that is,
   FETCH should be viewed as poking the server to include the desired
   data along with any other data it wishes to transmit to the client).
   There are three major categories of data which may be fetched.



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

   The second category is that data which describes the composition and
   delivery information of a message; that is, information such as the
   message sender, recipient lists, message-ID, subject, etc.  This is
   the information which is stored in the message header in RFC 822
   format message and is traditionally called the "envelope".  [Note:
   this should not be confused with the SMTP (RFC 821) envelope, which
   is strictly limited to delivery information.]  IMAP3 defines a
   structured and unambiguous representation for the envelope which is
   particularly nice for Lisp-based parsers.  A client can use the
   envelope for operations such as replying and not worry about RFC 822
   at all.  Envelopes are discussed in more detail below.  The first and
   second category data can be fetched together by using the macro-fetch
   word "ALL"; that is, "ALL" expands to "(FLAGS INTERNALDATE
   RFC822.SIZE ENVELOPE)".

   The third category is that data which is intended for direct human
   viewing.  The present RFC 822 based IMAP3 defines three such items:
   RFC822.HEADER, RFC822.TEXT, and RFC822 (the latter being the two
   former appended together in a single text string).  Fetching "RFC822"
   is equivalent to typing the RFC 822 representation of the message as
   stored on the mailbox without any filtering or processing.

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

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

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



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

A Typical Scenario

        Client                          Server
        ------                          ------
                                    {Wait for Connection}
    {Open Connection}        -->
                                <-- * OK IMAP3 Server Ready
                                    {Wait for command}
    A001 SUPPORTED.VERSIONS   -->
                                <-- * SUPPORTED.VERSIONS ((2 0 )
                                        (3 0 EIGHT.BIT.TRANSPARENT
                                             AUTO.SET.SEEN
                                             TAGGED.SOLICITED))
                                    A001 OK Supported Versions returned.
                                    {Wait for command}
    A002 SELECT.VERSION (3 0) -->
                                <-- A002 OK Version 3.0 Selected.
                                    {Wait for command}
    A002 SELECT.FEATURES TAGGED.SOLICITED -->
                                <-- A002 OK Features selected.
                                    {Wait for command}
    A003 LOGIN Fred Secret   -->
                                <-- A003 OK User Fred logged in
                                    {Wait for command}
    A004 SELECT INBOX        -->
                                <-- A004 FLAGS (Meeting Notice \Answered
                                             \Flagged \Deleted \Seen)
                                <-- A004 19 EXISTS
                                <-- A004 2 RECENT
                                <-- A004 OK Select complete
                                    {Wait for command}
    A005 FETCH 1:19 ALL      -->
                                <-- A005 1 Fetch (......)
                                        ...
                                <-- A005 18 Fetch (......)
                                <-- A005 19 Fetch (......)
                                <-- A005 OK Fetch complete
                                    {Wait for command}
    A006 FETCH 8 RFC822.TEXT -->
                                <-- A006 8 Fetch (RFC822.TEXT {893}
                                       ...893 characters of text...
                                <-- )
                                <-- A006 OK Fetch complete
                                    {Wait for command}




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    A007 STORE 8 +Flags \Deleted -->
                                <-- A007 8 Store (Flags (\Deleted
                                               \Seen))
                                <-- A007 OK Store complete
                                    {Wait for command}
    A008 EXPUNGE             -->
                                <-- A008 19 EXISTS
                                <-- A008 8 EXPUNGE
                                <-- A008 18 EXISTS
                                <-- A008 Expunge complete
                                    {Wait for command}
    A009 LOGOUT              -->
                                <-- A009 BYE IMAP3 server quitting
                                <-- A009 OK Logout complete
    {Close Connection}       --><-- {Close connection}
                                    {Go back to start}

   A more complex scenario produced by a pipelining multiprocess client.

        Client                          Server
        ------                          ------
                                    {Wait for Connection}
    {Open session as above}
                                <-- A004 19 EXISTS
                                <-- A004 2 RECENT
                                <-- A004 OK Select complete
                                    {Wait for command}
    A005 SEARCH RECENT       -->
                                <-- A005 SEARCH (18 19) (RECENT)
                                <---A005 OK Search complete
    A006 FETCH 18:19 ALL RFC822.TEXT
    A007 STORE 18:19 +FLAGS (\SEEN)
    A008 FETCH 1:17 ALL      -->
                                <-- A006 18 Fetch (... RFC822.TEXT ...)
    A009 STORE 18 +FLAGS (\DELETED)
                                <-- A006 19 Fetch (... RFC822.TEXT ...)
                                <-- A006 OK Fetch complete
                                <-- A007 18 STORE (Flags (\Seen))
    A010 STORE 19 +FLAGS (\DELETED)
                                <-- A007 19 STORE (Flags (\Seen))
                                <-- A007 OK Store complete
                                <-- A008 1 Fetch (......)
                                       ...
                                <-- A008 16 Fetch (......)
                                <-- A008 17 Fetch (......)
                                <-- A008 OK Fetch complete
                                <-- A009 18 STORE (Flags (\Seen
                                                          \Deleted))



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                                <-- A009 OK Store complete
                                <-- A010 19 STORE (Flags (\Seen
                                                          \Deleted))
                                <-- A010 OK Store complete
                                    {Wait for command}
                                <-- * EXISTS 23
                                <-- * RECENT 4
                                <-- * SEARCH (20 21 22 23) (RECENT)
   A011 FETCH 20:23 ALL RFC822.TEXT

Conventions

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

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

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

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

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

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

      A SP is the ASCII space character.

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

   One of the most common fields in the IMAP3 protocol is a STRING,
   which may be an ATOM, QUOTED-STRING (a sequence of CHARACTERs inside
   double-quotes), or a LITERAL.  A literal consists of an open brace
   ("{"), a number, a close brace ("}"), a CRLF, and then an ASCII-
   STRING of n characters, where n is the value of the number inside the
   brace. In general, a string should be represented as an ATOM or
   QUOTED-STRING if at all possible.  The semantics for QUOTED-STRING or
   LITERAL are checked before those for ATOM; therefore an ATOM used in
   a STRING may only contain CHARACTERs.  Literals are most often sent
   from the server to the client; in the rare case of a client to server
   literal there is a special consideration (see the "+ text" response
   below).

   Another important field is the SEQUENCE, which identifies a set of



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

Definitions of Commands and Responses

   Summary of Commands and Responses

Commands:
       tag NOOP
       tag LOGIN user password
       tag LOGOUT
       tag SELECT mailbox
       tag CHECK
       tag EXPUNGE
       tag COPY sequence mailbox
       tag FETCH sequence data
       tag STORE sequence data value
       tag SEARCH criteria
       tag BBOARD bboard
       tag FIND (BBOARDS / MAILBOXES) pattern
       tag READONLY
       tag READWRITE
       tag SELECT.VERSION (major_version minor_version)
       tag SELECT.FEATURES features
       tag SUPPORTED.VERSIONS
       tag FLAGS
       tag SET.FLAGS

Responses (can be either solicited or unsolicited):
       */tag FLAGS flag_list
       */tag SEARCH (numbers) (criteria)
       */tag EXISTS
       */tag RECENT
       */tag EXPUNGE
       */tag STORE data
       */tag FETCH data
       */tag BBOARD bboard_name
       */tag MAILBOX non_inbox_mailbox_name
       */tag SUPPORTED.VERSIONS version_data
       */tag READONLY
       */tag READWRITE
       */tag OK text
       */tag NO text
       */tag BAD text



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       */tag BYE text

Responses (can only be solicited):
       tag COPY message_number

Responses (can only be unsolicited):
       + text

Commands

   tag NOOP

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

   tag LOGIN user password

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

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

   tag LOGOUT

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

   tag SELECT mailbox

      The SELECT command selects a particular mailbox.  The server must
      check that the user is permitted read access to this mailbox.
      Prior to returning an OK to the client, the server must send an
      solicited FLAGS and <n> EXISTS response to the client giving the
      flags list for this mailbox (simply the system flags if this
      mailbox doesn't have any special flags) and the number of messages
      in the mailbox.  It is also recommended that the server send a <n>
      RECENT unsolicited response to the client for the benefit of
      clients which make use of the number of new messages in a mailbox.
      It is further recommended that servers should send an unsolicited
      READONLY message if the mailbox that has been selected is not



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      writable by the user.

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

      The default mailbox for the SELECT command is INBOX, which is a
      special name reserved to mean "the primary mailbox for this user
      on this server".  The format of other mailbox names is operating
      system dependent (as of this writing, it reflects the path of the
      mailbox on the current servers), though it could reflect any
      server-specific naming convention for the namespace of mailboxes.
      Such a namespace need not and should not be viewed as being
      equivalent or linked to the server machine's file system.

      EXAMPLES: A002 SELECT INBOX  ;; selects the default mailbox.
                A002 197 EXISTS    ;; server says 197 messages in INBOX
                A002 5 RECENT      ;; server says 5 are recent.
                A002 OK Select complete.
      or
                A003 SELECT /usr/fred/my-mail.txt
                 ;; select a different user specified mailbox.
                ...

   tag CHECK

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

   tag EXPUNGE

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



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