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RFC 2060


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Internet Message Access Protocol - Version 4rev1

Part 1 of 3, p. 1 to 17
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Obsoleted by:    3501
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Network Working Group                                        M. Crispin
Request for Comments: 2060                     University of Washington
Obsoletes: 1730                                           December 1996
Category: Standards Track


            INTERNET MESSAGE ACCESS PROTOCOL - VERSION 4rev1

Status of this Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.

Abstract

   The Internet Message Access Protocol, Version 4rev1 (IMAP4rev1)
   allows a client to access and manipulate electronic mail messages on
   a server.  IMAP4rev1 permits manipulation of remote message folders,
   called "mailboxes", in a way that is functionally equivalent to local
   mailboxes.  IMAP4rev1 also provides the capability for an offline
   client to resynchronize with the server (see also [IMAP-DISC]).

   IMAP4rev1 includes operations for creating, deleting, and renaming
   mailboxes; checking for new messages; permanently removing messages;
   setting and clearing flags; [RFC-822] and [MIME-IMB] parsing;
   searching; and selective fetching of message attributes, texts, and
   portions thereof.  Messages in IMAP4rev1 are accessed by the use of
   numbers.  These numbers are either message sequence numbers or unique
   identifiers.

   IMAP4rev1 supports a single server.  A mechanism for accessing
   configuration information to support multiple IMAP4rev1 servers is
   discussed in [ACAP].

   IMAP4rev1 does not specify a means of posting mail; this function is
   handled by a mail transfer protocol such as [SMTP].

   IMAP4rev1 is designed to be upwards compatible from the [IMAP2] and
   unpublished IMAP2bis protocols.  In the course of the evolution of
   IMAP4rev1, some aspects in the earlier protocol have become obsolete.
   Obsolete commands, responses, and data formats which an IMAP4rev1
   implementation may encounter when used with an earlier implementation
   are described in [IMAP-OBSOLETE].

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   Other compatibility issues with IMAP2bis, the most common variant of
   the earlier protocol, are discussed in [IMAP-COMPAT].  A full
   discussion of compatibility issues with rare (and presumed extinct)
   variants of [IMAP2] is in [IMAP-HISTORICAL]; this document is
   primarily of historical interest.

Table of Contents

 IMAP4rev1 Protocol Specification .................................    4
 1.      How to Read This Document ................................    4
 1.1.    Organization of This Document ............................    4
 1.2.    Conventions Used in This Document ........................    4
 2.      Protocol Overview ........................................    5
 2.1.    Link Level ...............................................    5
 2.2.    Commands and Responses ...................................    6
 2.2.1.  Client Protocol Sender and Server Protocol Receiver ......    6
 2.2.2.  Server Protocol Sender and Client Protocol Receiver ......    7
 2.3.    Message Attributes .......................................    7
 2.3.1.  Message Numbers ..........................................    7
 2.3.1.1.        Unique Identifier (UID) Message Attribute ........    7
 2.3.1.2.        Message Sequence Number Message Attribute ........    9
 2.3.2.  Flags Message Attribute ...................................   9
 2.3.3.  Internal Date Message Attribute ..........................   10
 2.3.4.  [RFC-822] Size Message Attribute .........................   11
 2.3.5.  Envelope Structure Message Attribute .....................   11
 2.3.6.  Body Structure Message Attribute .........................   11
 2.4.    Message Texts ............................................   11
 3.      State and Flow Diagram ...................................   11
 3.1.    Non-Authenticated State ..................................   11
 3.2.    Authenticated State ......................................   11
 3.3.    Selected State ...........................................   12
 3.4.    Logout State .............................................   12
 4.      Data Formats .............................................   12
 4.1.    Atom .....................................................   13
 4.2.    Number ...................................................   13
 4.3.    String ....................................................  13
 4.3.1.  8-bit and Binary Strings .................................   13
 4.4.    Parenthesized List .......................................   14
 4.5.    NIL ......................................................   14
 5.      Operational Considerations ...............................   14
 5.1.    Mailbox Naming ...........................................   14
 5.1.1.  Mailbox Hierarchy Naming .................................   14
 5.1.2.  Mailbox Namespace Naming Convention ......................   14
 5.1.3.  Mailbox International Naming Convention ..................   15
 5.2.    Mailbox Size and Message Status Updates ..................   16
 5.3.    Response when no Command in Progress .....................   16
 5.4.    Autologout Timer .........................................   16
 5.5.    Multiple Commands in Progress ............................   17

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 6.      Client Commands ..........................................   17
 6.1.    Client Commands - Any State ..............................   18
 6.1.1.  CAPABILITY Command .......................................   18
 6.1.2.  NOOP Command .............................................   19
 6.1.3.  LOGOUT Command ...........................................   20
 6.2.    Client Commands - Non-Authenticated State ................   20
 6.2.1.  AUTHENTICATE Command .....................................   21
 6.2.2.  LOGIN Command ............................................   22
 6.3.    Client Commands - Authenticated State ....................   22
 6.3.1.  SELECT Command ...........................................   23
 6.3.2.  EXAMINE Command ..........................................   24
 6.3.3.  CREATE Command ...........................................   25
 6.3.4.  DELETE Command ...........................................   26
 6.3.5.  RENAME Command ...........................................   27
 6.3.6.  SUBSCRIBE Command ........................................   29
 6.3.7.  UNSUBSCRIBE Command ......................................   30
 6.3.8.  LIST Command .............................................   30
 6.3.9.  LSUB Command .............................................   32
 6.3.10. STATUS Command ...........................................   33
 6.3.11. APPEND Command ...........................................   34
 6.4.    Client Commands - Selected State .........................   35
 6.4.1.  CHECK Command ............................................   36
 6.4.2.  CLOSE Command ............................................   36
 6.4.3.  EXPUNGE Command ..........................................   37
 6.4.4.  SEARCH Command ...........................................   37
 6.4.5.  FETCH Command ............................................   41
 6.4.6.  STORE Command ............................................   45
 6.4.7.  COPY Command .............................................   46
 6.4.8.  UID Command ..............................................   47
 6.5.    Client Commands - Experimental/Expansion .................   48
 6.5.1.  X<atom> Command ..........................................   48
 7.      Server Responses .........................................   48
 7.1.    Server Responses - Status Responses ......................   49
 7.1.1.  OK Response ..............................................   51
 7.1.2.  NO Response ..............................................   51
 7.1.3.  BAD Response .............................................   52
 7.1.4.  PREAUTH Response .........................................   52
 7.1.5.  BYE Response .............................................   52
 7.2.    Server Responses - Server and Mailbox Status .............   53
 7.2.1.  CAPABILITY Response ......................................   53
 7.2.2.  LIST Response .............................................  54
 7.2.3.  LSUB Response ............................................   55
 7.2.4   STATUS Response ..........................................   55
 7.2.5.  SEARCH Response ..........................................   55
 7.2.6.  FLAGS Response ...........................................   56
 7.3.    Server Responses - Mailbox Size ..........................   56
 7.3.1.  EXISTS Response ..........................................   56
 7.3.2.  RECENT Response ..........................................   57

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 7.4.    Server Responses - Message Status ........................   57
 7.4.1.  EXPUNGE Response .........................................   57
 7.4.2.  FETCH Response ...........................................   58
 7.5.    Server Responses - Command Continuation Request ..........   63
 8.      Sample IMAP4rev1 connection ..............................   63
 9.      Formal Syntax ............................................   64
 10.     Author's Note ............................................   74
 11.     Security Considerations ..................................   74
 12.     Author's Address .........................................   75
 Appendices .......................................................   76
 A.      References ...............................................   76
 B.      Changes from RFC 1730 ....................................   77
 C.      Key Word Index ...........................................   79


IMAP4rev1 Protocol Specification

1.      How to Read This Document

1.1.    Organization of This Document

   This document is written from the point of view of the implementor of
   an IMAP4rev1 client or server.  Beyond the protocol overview in
   section 2, it is not optimized for someone trying to understand the
   operation of the protocol.  The material in sections 3 through 5
   provides the general context and definitions with which IMAP4rev1
   operates.

   Sections 6, 7, and 9 describe the IMAP commands, responses, and
   syntax, respectively.  The relationships among these are such that it
   is almost impossible to understand any of them separately.  In
   particular, do not attempt to deduce command syntax from the command
   section alone; instead refer to the Formal Syntax section.

1.2.    Conventions Used in This Document

   In examples, "C:" and "S:" indicate lines sent by the client and
   server respectively.

   The following terms are used in this document to signify the
   requirements of this specification.

   1) MUST, or the adjective REQUIRED, means that the definition is
      an absolute requirement of the specification.

   2) MUST NOT that the definition is an absolute prohibition of the
      specification.

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   3) SHOULD means that there may exist valid reasons in particular
      circumstances to ignore a particular item, but the full
      implications MUST be understood and carefully weighed before
      choosing a different course.

   4) SHOULD NOT means that there may exist valid reasons in
      particular circumstances when the particular behavior is
      acceptable or even useful, but the full implications SHOULD be
      understood and the case carefully weighed before implementing
      any behavior described with this label.

   5) MAY, or the adjective OPTIONAL, means that an item is truly
      optional.  One vendor may choose to include the item because a
      particular marketplace requires it or because the vendor feels
      that it enhances the product while another vendor may omit the
      same item.  An implementation which does not include a
      particular option MUST be prepared to interoperate with another
      implementation which does include the option.

      "Can" is used instead of "may" when referring to a possible
      circumstance or situation, as opposed to an optional facility of
      the protocol.

      "User" is used to refer to a human user, whereas "client" refers
      to the software being run by the user.

      "Connection" refers to the entire sequence of client/server
      interaction from the initial establishment of the network
      connection until its termination.  "Session" refers to the
      sequence of client/server interaction from the time that a mailbox
      is selected (SELECT or EXAMINE command) until the time that
      selection ends (SELECT or EXAMINE of another mailbox, CLOSE
      command, or connection termination).

       Characters are 7-bit US-ASCII unless otherwise specified.  Other
       character sets are indicated using a "CHARSET", as described in
       [MIME-IMT] and defined in [CHARSET].  CHARSETs have important
       additional semantics in addition to defining character set; refer
       to these documents for more detail.

2.      Protocol Overview

2.1.    Link Level

   The IMAP4rev1 protocol assumes a reliable data stream such as
   provided by TCP.  When TCP is used, an IMAP4rev1 server listens on
   port 143.

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2.2.    Commands and Responses

   An IMAP4rev1 connection consists of the establishment of a
   client/server network connection, an initial greeting from the
   server, and client/server interactions.  These client/server
   interactions consist of a client command, server data, and a server
   completion result response.

   All interactions transmitted by client and server are in the form of
   lines; that is, strings that end with a CRLF.  The protocol receiver
   of an IMAP4rev1 client or server is either reading a line, or is
   reading a sequence of octets with a known count followed by a line.

2.2.1.  Client Protocol Sender and Server Protocol Receiver

   The client command begins an operation.  Each client command is
   prefixed with an identifier (typically a short alphanumeric string,
   e.g. A0001, A0002, etc.) called a "tag".  A different tag is
   generated by the client for each command.

   There are two cases in which a line from the client does not
   represent a complete command.  In one case, a command argument is
   quoted with an octet count (see the description of literal in String
   under Data Formats); in the other case, the command arguments require
   server feedback (see the AUTHENTICATE command).  In either case, the
   server sends a command continuation request response if it is ready
   for the octets (if appropriate) and the remainder of the command.
   This response is prefixed with the token "+".

      Note: If, instead, the server detected an error in the command, it
      sends a BAD completion response with tag matching the command (as
      described below) to reject the command and prevent the client from
      sending any more of the command.

      It is also possible for the server to send a completion response
      for some other command (if multiple commands are in progress), or
      untagged data.  In either case, the command continuation request
      is still pending; the client takes the appropriate action for the
      response, and reads another response from the server.  In all
      cases, the client MUST send a complete command (including
      receiving all command continuation request responses and command
      continuations for the command) before initiating a new command.

   The protocol receiver of an IMAP4rev1 server reads a command line
   from the client, parses the command and its arguments, and transmits
   server data and a server command completion result response.

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2.2.2.  Server Protocol Sender and Client Protocol Receiver

   Data transmitted by the server to the client and status responses
   that do not indicate command completion are prefixed with the token
   "*", and are called untagged responses.

   Server data MAY be sent as a result of a client command, or MAY be
   sent unilaterally by the server.  There is no syntactic difference
   between server data that resulted from a specific command and server
   data that were sent unilaterally.

   The server completion result response indicates the success or
   failure of the operation.  It is tagged with the same tag as the
   client command which began the operation.  Thus, if more than one
   command is in progress, the tag in a server completion response
   identifies the command to which the response applies.  There are
   three possible server completion responses: OK (indicating success),
   NO (indicating failure), or BAD (indicating protocol error such as
   unrecognized command or command syntax error).

   The protocol receiver of an IMAP4rev1 client reads a response line
   from the server.  It then takes action on the response based upon the
   first token of the response, which can be a tag, a "*", or a "+".

   A client MUST be prepared to accept any server response at all times.
   This includes server data that was not requested.  Server data SHOULD
   be recorded, so that the client can reference its recorded copy
   rather than sending a command to the server to request the data.  In
   the case of certain server data, the data MUST be recorded.

   This topic is discussed in greater detail in the Server Responses
   section.

2.3.    Message Attributes

   In addition to message text, each message has several attributes
   associated with it.  These attributes may be retrieved individually
   or in conjunction with other attributes or message texts.

2.3.1.  Message Numbers

   Messages in IMAP4rev1 are accessed by one of two numbers; the unique
   identifier and the message sequence number.

2.3.1.1.        Unique Identifier (UID) Message Attribute

   A 32-bit value assigned to each message, which when used with the
   unique identifier validity value (see below) forms a 64-bit value

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   that is permanently guaranteed not to refer to any other message in
   the mailbox.  Unique identifiers are assigned in a strictly ascending
   fashion in the mailbox; as each message is added to the mailbox it is
   assigned a higher UID than the message(s) which were added
   previously.

   Unlike message sequence numbers, unique identifiers are not
   necessarily contiguous.  Unique identifiers also persist across
   sessions.  This permits a client to resynchronize its state from a
   previous session with the server (e.g. disconnected or offline access
   clients); this is discussed further in [IMAP-DISC].

   Associated with every mailbox is a unique identifier validity value,
   which is sent in an UIDVALIDITY response code in an OK untagged
   response at mailbox selection time.  If unique identifiers from an
   earlier session fail to persist to this session, the unique
   identifier validity value MUST be greater than the one used in the
   earlier session.

      Note: Unique identifiers MUST be strictly ascending in the mailbox
      at all times.  If the physical message store is re-ordered by a
      non-IMAP agent, this requires that the unique identifiers in the
      mailbox be regenerated, since the former unique identifers are no
      longer strictly ascending as a result of the re-ordering.  Another
      instance in which unique identifiers are regenerated is if the
      message store has no mechanism to store unique identifiers.
      Although this specification recognizes that this may be
      unavoidable in certain server environments, it STRONGLY ENCOURAGES
      message store implementation techniques that avoid this problem.

      Another cause of non-persistance is if the mailbox is deleted and
      a new mailbox with the same name is created at a later date, Since
      the name is the same, a client may not know that this is a new
      mailbox unless the unique identifier validity is different.  A
      good value to use for the unique identifier validity value is a
      32-bit representation of the creation date/time of the mailbox.
      It is alright to use a constant such as 1, but only if it
      guaranteed that unique identifiers will never be reused, even in
      the case of a mailbox being deleted (or renamed) and a new mailbox
      by the same name created at some future time.

   The unique identifier of a message MUST NOT change during the
   session, and SHOULD NOT change between sessions.  However, if it is
   not possible to preserve the unique identifier of a message in a
   subsequent session, each subsequent session MUST have a new unique
   identifier validity value that is larger than any that was used
   previously.

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2.3.1.2.        Message Sequence Number Message Attribute

   A relative position from 1 to the number of messages in the mailbox.
   This position MUST be ordered by ascending unique identifier.  As
   each new message is added, it is assigned a message sequence number
   that is 1 higher than the number of messages in the mailbox before
   that new message was added.

   Message sequence numbers can be reassigned during the session.  For
   example, when a message is permanently removed (expunged) from the
   mailbox, the message sequence number for all subsequent messages is
   decremented.  Similarly, a new message can be assigned a message
   sequence number that was once held by some other message prior to an
   expunge.

   In addition to accessing messages by relative position in the
   mailbox, message sequence numbers can be used in mathematical
   calculations.  For example, if an untagged "EXISTS 11" is received,
   and previously an untagged "8 EXISTS" was received, three new
   messages have arrived with message sequence numbers of 9, 10, and 11.
   Another example; if message 287 in a 523 message mailbox has UID
   12345, there are exactly 286 messages which have lesser UIDs and 236
   messages which have greater UIDs.

2.3.2.  Flags Message Attribute

   A list of zero or more named tokens associated with the message.  A
   flag is set by its addition to this list, and is cleared by its
   removal.  There are two types of flags in IMAP4rev1.  A flag of
   either type may be permanent or session-only.

   A system flag is a flag name that is pre-defined in this
   specification.  All system flags begin with "\".  Certain system
   flags (\Deleted and \Seen) have special semantics described
   elsewhere.  The currently-defined system flags are:

        \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

        \Draft      Message has not completed composition (marked as a
                    draft).

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        \Recent     Message is "recently" arrived in this mailbox.  This
                    session is the first session to have been notified
                    about this message; subsequent sessions will not see
                    \Recent set for this message.  This flag can not be
                    altered by the client.

                    If it is not possible to determine whether or not
                    this session is the first session to be notified
                    about a message, then that message SHOULD be
                    considered recent.

                    If multiple connections have the same mailbox
                    selected simultaneously, it is undefined which of
                    these connections will see newly-arrives messages
                    with \Recent set and which will see it without
                    \Recent set.

      A keyword is defined by the server implementation.  Keywords do
      not begin with "\".  Servers MAY permit the client to define new
      keywords in the mailbox (see the description of the
      PERMANENTFLAGS response code for more information).

      A flag may be permanent or session-only on a per-flag basis.
      Permanent flags are those which the client can add or remove
      from the message flags permanently; that is, subsequent sessions
      will see any change in permanent flags.  Changes to session
      flags are valid only in that session.

      Note: The \Recent system flag is a special case of a
      session flag.  \Recent can not be used as an argument in a
      STORE command, and thus can not be changed at all.

2.3.3.  Internal Date Message Attribute

   The internal date and time of the message on the server.  This is not
   the date and time in the [RFC-822] header, but rather a date and time
   which reflects when the message was received.  In the case of
   messages delivered via [SMTP], this SHOULD be the date and time of
   final delivery of the message as defined by [SMTP].  In the case of
   messages delivered by the IMAP4rev1 COPY command, this SHOULD be the
   internal date and time of the source message.  In the case of
   messages delivered by the IMAP4rev1 APPEND command, this SHOULD be
   the date and time as specified in the APPEND command description.
   All other cases are implementation defined.

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2.3.4.  [RFC-822] Size Message Attribute

   The number of octets in the message, as expressed in [RFC-822]
   format.

2.3.5.  Envelope Structure Message Attribute

   A parsed representation of the [RFC-822] envelope information (not to
   be confused with an [SMTP] envelope) of the message.

2.3.6.  Body Structure Message Attribute

   A parsed representation of the [MIME-IMB] body structure information
   of the message.

2.4.    Message Texts

   In addition to being able to fetch the full [RFC-822] text of a
   message, IMAP4rev1 permits the fetching of portions of the full
   message text.  Specifically, it is possible to fetch the [RFC-822]
   message header, [RFC-822] message body, a [MIME-IMB] body part, or a
   [MIME-IMB] header.

3.      State and Flow Diagram

   An IMAP4rev1 server is in one of four states.  Most commands are
   valid in only certain states.  It is a protocol error for the client
   to attempt a command while the command is in an inappropriate state.
   In this case, a server will respond with a BAD or NO (depending upon
   server implementation) command completion result.

3.1.    Non-Authenticated State

   In non-authenticated state, the client MUST supply authentication
   credentials before most commands will be permitted.  This state is
   entered when a connection starts unless the connection has been pre-
   authenticated.

3.2.    Authenticated State

   In authenticated state, the client is authenticated and MUST select a
   mailbox to access before commands that affect messages will be
   permitted.  This state is entered when a pre-authenticated connection
   starts, when acceptable authentication credentials have been
   provided, or after an error in selecting a mailbox.

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3.3.    Selected State

   In selected state, a mailbox has been selected to access.  This state
   is entered when a mailbox has been successfully selected.

3.4.    Logout State

   In logout state, the connection is being terminated, and the server
   will close the connection.  This state can be entered as a result of
   a client request or by unilateral server decision.

            +--------------------------------------+
            |initial connection and server greeting|
            +--------------------------------------+
                      || (1)       || (2)        || (3)
                      VV           ||            ||
            +-----------------+    ||            ||
            |non-authenticated|    ||            ||
            +-----------------+    ||            ||
             || (7)   || (4)       ||            ||
             ||       VV           VV            ||
             ||     +----------------+           ||
             ||     | authenticated  |<=++       ||
             ||     +----------------+  ||       ||
             ||       || (7)   || (5)   || (6)   ||
             ||       ||       VV       ||       ||
             ||       ||    +--------+  ||       ||
             ||       ||    |selected|==++       ||
             ||       ||    +--------+           ||
             ||       ||       || (7)            ||
             VV       VV       VV                VV
            +--------------------------------------+
            |     logout and close connection      |
            +--------------------------------------+

         (1) connection without pre-authentication (OK greeting)
         (2) pre-authenticated connection (PREAUTH greeting)
         (3) rejected connection (BYE greeting)
         (4) successful LOGIN or AUTHENTICATE command
         (5) successful SELECT or EXAMINE command
         (6) CLOSE command, or failed SELECT or EXAMINE command
         (7) LOGOUT command, server shutdown, or connection closed

4.      Data Formats

   IMAP4rev1 uses textual commands and responses.  Data in IMAP4rev1 can
   be in one of several forms: atom, number, string, parenthesized list,
   or NIL.

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4.1.    Atom

   An atom consists of one or more non-special characters.

4.2.    Number

   A number consists of one or more digit characters, and represents a
   numeric value.

4.3.    String

   A string is in one of two forms: literal and quoted string.  The
   literal form is the general form of string.  The quoted string form
   is an alternative that avoids the overhead of processing a literal at
   the cost of limitations of characters that can be used in a quoted
   string.

   A literal is a sequence of zero or more octets (including CR and LF),
   prefix-quoted with an octet count in the form of an open brace ("{"),
   the number of octets, close brace ("}"), and CRLF.  In the case of
   literals transmitted from server to client, the CRLF is immediately
   followed by the octet data.  In the case of literals transmitted from
   client to server, the client MUST wait to receive a command
   continuation request (described later in this document) before
   sending the octet data (and the remainder of the command).

   A quoted string is a sequence of zero or more 7-bit characters,
   excluding CR and LF, with double quote (<">) characters at each end.

   The empty string is represented as either "" (a quoted string with
   zero characters between double quotes) or as {0} followed by CRLF (a
   literal with an octet count of 0).

      Note: Even if the octet count is 0, a client transmitting a
      literal MUST wait to receive a command continuation request.

4.3.1.  8-bit and Binary Strings

   8-bit textual and binary mail is supported through the use of a
   [MIME-IMB] content transfer encoding.  IMAP4rev1 implementations MAY
   transmit 8-bit or multi-octet characters in literals, but SHOULD do
   so only when the [CHARSET] is identified.

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   Although a BINARY body encoding is defined, unencoded binary strings
   are not permitted.  A "binary string" is any string with NUL
   characters.  Implementations MUST encode binary data into a textual
   form such as BASE64 before transmitting the data.  A string with an
   excessive amount of CTL characters MAY also be considered to be
   binary.

4.4.    Parenthesized List

   Data structures are represented as a "parenthesized list"; a sequence
   of data items, delimited by space, and bounded at each end by
   parentheses.  A parenthesized list can contain other parenthesized
   lists, using multiple levels of parentheses to indicate nesting.

   The empty list is represented as () -- a parenthesized list with no
   members.

4.5.    NIL

   The special atom "NIL" represents the non-existence of a particular
   data item that is represented as a string or parenthesized list, as
   distinct from the empty string "" or the empty parenthesized list ().

5.      Operational Considerations

5.1.    Mailbox Naming

   The interpretation of mailbox names is implementation-dependent.
   However, the case-insensitive mailbox name INBOX is a special name
   reserved to mean "the primary mailbox for this user on this server".

5.1.1.  Mailbox Hierarchy Naming

   If it is desired to export hierarchical mailbox names, mailbox names
   MUST be left-to-right hierarchical using a single character to
   separate levels of hierarchy.  The same hierarchy separator character
   is used for all levels of hierarchy within a single name.

5.1.2.  Mailbox Namespace Naming Convention

   By convention, the first hierarchical element of any mailbox name
   which begins with "#" identifies the "namespace" of the remainder of
   the name.  This makes it possible to disambiguate between different
   types of mailbox stores, each of which have their own namespaces.

Top      ToC       Page 15 
      For example, implementations which offer access to USENET
      newsgroups MAY use the "#news" namespace to partition the USENET
      newsgroup namespace from that of other mailboxes.  Thus, the
      comp.mail.misc newsgroup would have an mailbox name of
      "#news.comp.mail.misc", and the name "comp.mail.misc" could refer
      to a different object (e.g. a user's private mailbox).

5.1.3.  Mailbox International Naming Convention

   By convention, international mailbox names are specified using a
   modified version of the UTF-7 encoding described in [UTF-7].  The
   purpose of these modifications is to correct the following problems
   with UTF-7:

      1) UTF-7 uses the "+" character for shifting; this conflicts with
         the common use of "+" in mailbox names, in particular USENET
         newsgroup names.

      2) UTF-7's encoding is BASE64 which uses the "/" character; this
         conflicts with the use of "/" as a popular hierarchy delimiter.

      3) UTF-7 prohibits the unencoded usage of "\"; this conflicts with
         the use of "\" as a popular hierarchy delimiter.

      4) UTF-7 prohibits the unencoded usage of "~"; this conflicts with
         the use of "~" in some servers as a home directory indicator.

      5) UTF-7 permits multiple alternate forms to represent the same
         string; in particular, printable US-ASCII chararacters can be
         represented in encoded form.

   In modified UTF-7, printable US-ASCII characters except for "&"
   represent themselves; that is, characters with octet values 0x20-0x25
   and 0x27-0x7e.  The character "&" (0x26) is represented by the two-
   octet sequence "&-".

   All other characters (octet values 0x00-0x1f, 0x7f-0xff, and all
   Unicode 16-bit octets) are represented in modified BASE64, with a
   further modification from [UTF-7] that "," is used instead of "/".
   Modified BASE64 MUST NOT be used to represent any printing US-ASCII
   character which can represent itself.

   "&" is used to shift to modified BASE64 and "-" to shift back to US-
   ASCII.  All names start in US-ASCII, and MUST end in US-ASCII (that
   is, a name that ends with a Unicode 16-bit octet MUST end with a "-
   ").

Top      ToC       Page 16 
      For example, here is a mailbox name which mixes English, Japanese,
      and Chinese text: ~peter/mail/&ZeVnLIqe-/&U,BTFw-

5.2.    Mailbox Size and Message Status Updates

   At any time, a server can send data that the client did not request.
   Sometimes, such behavior is REQUIRED.  For example, agents other than
   the server MAY add messages to the mailbox (e.g. new mail delivery),
   change the flags of message in the mailbox (e.g. simultaneous access
   to the same mailbox by multiple agents), or even remove messages from
   the mailbox.  A server MUST send mailbox size updates automatically
   if a mailbox size change is observed during the processing of a
   command.  A server SHOULD send message flag updates automatically,
   without requiring the client to request such updates explicitly.
   Special rules exist for server notification of a client about the
   removal of messages to prevent synchronization errors; see the
   description of the EXPUNGE response for more detail.

   Regardless of what implementation decisions a client makes on
   remembering data from the server, a client implementation MUST record
   mailbox size updates.  It MUST NOT assume that any command after
   initial mailbox selection will return the size of the mailbox.

5.3.    Response when no Command in Progress

   Server implementations are permitted to send an untagged response
   (except for EXPUNGE) while there is no command in progress.  Server
   implementations that send such responses MUST deal with flow control
   considerations.  Specifically, they MUST either (1) verify that the
   size of the data does not exceed the underlying transport's available
   window size, or (2) use non-blocking writes.

5.4.    Autologout Timer

   If a server has an inactivity autologout timer, that timer MUST be of
   at least 30 minutes' duration.  The receipt of ANY command from the
   client during that interval SHOULD suffice to reset the autologout
   timer.

Top      ToC       Page 17 
5.5.    Multiple Commands in Progress

   The client MAY send another command without waiting for the
   completion result response of a command, subject to ambiguity rules
   (see below) and flow control constraints on the underlying data
   stream.  Similarly, a server MAY begin processing another command
   before processing the current command to completion, subject to
   ambiguity rules.  However, any command continuation request responses
   and command continuations MUST be negotiated before any subsequent
   command is initiated.

   The exception is if an ambiguity would result because of a command
   that would affect the results of other commands.  Clients MUST NOT
   send multiple commands without waiting if an ambiguity would result.
   If the server detects a possible ambiguity, it MUST execute commands
   to completion in the order given by the client.

   The most obvious example of ambiguity is when a command would affect
   the results of another command; for example, a FETCH of a message's
   flags and a STORE of that same message's flags.

   A non-obvious ambiguity occurs with commands that permit an untagged
   EXPUNGE response (commands other than FETCH, STORE, and SEARCH),
   since an untagged EXPUNGE response can invalidate sequence numbers in
   a subsequent command.  This is not a problem for FETCH, STORE, or
   SEARCH commands because servers are prohibited from sending EXPUNGE
   responses while any of those commands are in progress.  Therefore, if
   the client sends any command other than FETCH, STORE, or SEARCH, it
   MUST wait for a response before sending a command with message
   sequence numbers.

   For example, the following non-waiting command sequences are invalid:

      FETCH + NOOP + STORE
      STORE + COPY + FETCH
      COPY + COPY
      CHECK + FETCH

   The following are examples of valid non-waiting command sequences:

      FETCH + STORE + SEARCH + CHECK
      STORE + COPY + EXPUNGE



(page 17 continued on part 2)

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