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INTERNET MESSAGE ACCESS PROTOCOL - VERSION 4rev1
draft-melnikov-rfc3501bis-00

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This is an older version of an Internet-Draft whose latest revision state is "Expired".
Author Alexey Melnikov
Last updated 2015-03-29
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draft-melnikov-rfc3501bis-00
Network Working Group                                   A. Melnikov, Ed.
Internet-Draft                                                 Isode Ltd
Obsoletes: 3501 (if approved)                             March 29, 2015
Intended status: Standards Track
Expires: September 30, 2015

            INTERNET MESSAGE ACCESS PROTOCOL - VERSION 4rev1
                    draft-melnikov-rfc3501bis-00.txt

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 mailboxes (remote
   message folders) in a way that is functionally equivalent to local
   folders.  IMAP4rev1 also provides the capability for an offline
   client to resynchronize with the server.

   IMAP4rev1 includes operations for creating, deleting, and renaming
   mailboxes, checking for new messages, permanently removing messages,
   setting and clearing flags, RFC 2822 and RFC 2045 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 RFC 2244.

   IMAP4rev1 does not specify a means of posting mail; this function is
   handled by a mail transfer protocol such as RFC 2821.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

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   This Internet-Draft will expire on September 30, 2015.

Copyright Notice

   Copyright (c) 2015 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
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   publication of this document.  Please review these documents
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   described in the Simplified BSD License.

   This document may contain material from IETF Documents or IETF
   Contributions published or made publicly available before November
   10, 2008.  The person(s) controlling the copyright in some of this
   material may not have granted the IETF Trust the right to allow
   modifications of such material outside the IETF Standards Process.
   Without obtaining an adequate license from the person(s) controlling
   the copyright in such materials, this document may not be modified
   outside the IETF Standards Process, and derivative works of it may
   not be created outside the IETF Standards Process, except to format
   it for publication as an RFC or to translate it into languages other
   than English.

Table of Contents

   1.  How to Read This Document . . . . . . . . . . . . . . . . . .   4
     1.1.  Organization of This Document . . . . . . . . . . . . . .   5
     1.2.  Conventions Used in This Document . . . . . . . . . . . .   5
     1.3.  Special Notes to Implementors . . . . . . . . . . . . . .   6
   2.  Protocol Overview . . . . . . . . . . . . . . . . . . . . . .   6
     2.1.  Link Level  . . . . . . . . . . . . . . . . . . . . . . .   6
     2.2.  Commands and Responses  . . . . . . . . . . . . . . . . .   7
       2.2.1.  Client Protocol Sender and Server Protocol Receiver .   7
       2.2.2.  Server Protocol Sender and Client Protocol Receiver .   8
     2.3.  Message Attributes  . . . . . . . . . . . . . . . . . . .   8
       2.3.1.  Message Numbers . . . . . . . . . . . . . . . . . . .   9
       2.3.2.  Flags Message Attribute . . . . . . . . . . . . . . .  11
       2.3.3.  Internal Date Message Attribute . . . . . . . . . . .  12
       2.3.4.  [RFC-2822] Size Message Attribute . . . . . . . . . .  12
       2.3.5.  Envelope Structure Message Attribute  . . . . . . . .  12
       2.3.6.  Body Structure Message Attribute  . . . . . . . . . .  13
     2.4.  Message Texts . . . . . . . . . . . . . . . . . . . . . .  13

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   3.  State and Flow Diagram  . . . . . . . . . . . . . . . . . . .  13
     3.1.  Not Authenticated State . . . . . . . . . . . . . . . . .  13
     3.2.  Authenticated State . . . . . . . . . . . . . . . . . . .  13
     3.3.  Selected State  . . . . . . . . . . . . . . . . . . . . .  13
     3.4.  Logout State  . . . . . . . . . . . . . . . . . . . . . .  14
   4.  Data Formats  . . . . . . . . . . . . . . . . . . . . . . . .  15
     4.1.  Atom  . . . . . . . . . . . . . . . . . . . . . . . . . .  16
     4.2.  Number  . . . . . . . . . . . . . . . . . . . . . . . . .  16
     4.3.  String  . . . . . . . . . . . . . . . . . . . . . . . . .  16
       4.3.1.  8-bit and Binary Strings  . . . . . . . . . . . . . .  16
     4.4.  Parenthesized List  . . . . . . . . . . . . . . . . . . .  17
     4.5.  NIL . . . . . . . . . . . . . . . . . . . . . . . . . . .  17
   5.  Operational Considerations  . . . . . . . . . . . . . . . . .  17
     5.1.  Mailbox Naming  . . . . . . . . . . . . . . . . . . . . .  17
       5.1.1.  Mailbox Hierarchy Naming  . . . . . . . . . . . . . .  18
       5.1.2.  Mailbox Namespace Naming Convention . . . . . . . . .  18
       5.1.3.  Mailbox International Naming Convention . . . . . . .  19
     5.2.  Mailbox Size and Message Status Updates . . . . . . . . .  20
     5.3.  Response when no Command in Progress  . . . . . . . . . .  21
     5.4.  Autologout Timer  . . . . . . . . . . . . . . . . . . . .  21
     5.5.  Multiple Commands in Progress . . . . . . . . . . . . . .  21
   6.  Client Commands . . . . . . . . . . . . . . . . . . . . . . .  22
     6.1.  Client Commands - Any State . . . . . . . . . . . . . . .  23
       6.1.1.  CAPABILITY Command  . . . . . . . . . . . . . . . . .  23
       6.1.2.  NOOP Command  . . . . . . . . . . . . . . . . . . . .  24
       6.1.3.  LOGOUT Command  . . . . . . . . . . . . . . . . . . .  25
     6.2.  Client Commands - Not Authenticated State . . . . . . . .  25
       6.2.1.  STARTTLS Command  . . . . . . . . . . . . . . . . . .  26
       6.2.2.  AUTHENTICATE Command  . . . . . . . . . . . . . . . .  27
       6.2.3.  LOGIN Command . . . . . . . . . . . . . . . . . . . .  29
     6.3.  Client Commands - Authenticated State . . . . . . . . . .  30
       6.3.1.  SELECT Command  . . . . . . . . . . . . . . . . . . .  30
       6.3.2.  EXAMINE Command . . . . . . . . . . . . . . . . . . .  32
       6.3.3.  CREATE Command  . . . . . . . . . . . . . . . . . . .  33
       6.3.4.  DELETE Command  . . . . . . . . . . . . . . . . . . .  34
       6.3.5.  RENAME Command  . . . . . . . . . . . . . . . . . . .  35
       6.3.6.  SUBSCRIBE Command . . . . . . . . . . . . . . . . . .  37
       6.3.7.  UNSUBSCRIBE Command . . . . . . . . . . . . . . . . .  38
       6.3.8.  LIST Command  . . . . . . . . . . . . . . . . . . . .  38
       6.3.9.  LSUB Command  . . . . . . . . . . . . . . . . . . . .  41
       6.3.10. STATUS Command  . . . . . . . . . . . . . . . . . . .  42
       6.3.11. APPEND Command  . . . . . . . . . . . . . . . . . . .  43
     6.4.  Client Commands - Selected State  . . . . . . . . . . . .  45
       6.4.1.  CHECK Command . . . . . . . . . . . . . . . . . . . .  45
       6.4.2.  CLOSE Command . . . . . . . . . . . . . . . . . . . .  45
       6.4.3.  EXPUNGE Command . . . . . . . . . . . . . . . . . . .  46
       6.4.4.  SEARCH Command  . . . . . . . . . . . . . . . . . . .  47
       6.4.5.  FETCH Command . . . . . . . . . . . . . . . . . . . .  50

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       6.4.6.  STORE Command . . . . . . . . . . . . . . . . . . . .  54
       6.4.7.  COPY Command  . . . . . . . . . . . . . . . . . . . .  55
       6.4.8.  UID Command . . . . . . . . . . . . . . . . . . . . .  56
     6.5.  Client Commands - Experimental/Expansion  . . . . . . . .  57
       6.5.1.  X<atom> Command . . . . . . . . . . . . . . . . . . .  57
   7.  Server Responses  . . . . . . . . . . . . . . . . . . . . . .  58
     7.1.  Server Responses - Status Responses . . . . . . . . . . .  59
       7.1.1.  OK Response . . . . . . . . . . . . . . . . . . . . .  60
       7.1.2.  NO Response . . . . . . . . . . . . . . . . . . . . .  61
       7.1.3.  BAD Response  . . . . . . . . . . . . . . . . . . . .  61
       7.1.4.  PREAUTH Response  . . . . . . . . . . . . . . . . . .  61
       7.1.5.  BYE Response  . . . . . . . . . . . . . . . . . . . .  62
     7.2.  Server Responses - Server and Mailbox Status  . . . . . .  62
       7.2.1.  CAPABILITY Response . . . . . . . . . . . . . . . . .  63
       7.2.2.  LIST Response . . . . . . . . . . . . . . . . . . . .  64
       7.2.3.  LSUB Response . . . . . . . . . . . . . . . . . . . .  65
       7.2.4.  STATUS Response . . . . . . . . . . . . . . . . . . .  65
       7.2.5.  SEARCH Response . . . . . . . . . . . . . . . . . . .  65
       7.2.6.  FLAGS Response  . . . . . . . . . . . . . . . . . . .  65
     7.3.  Server Responses - Mailbox Size . . . . . . . . . . . . .  66
       7.3.1.  EXISTS Response . . . . . . . . . . . . . . . . . . .  66
       7.3.2.  RECENT Response . . . . . . . . . . . . . . . . . . .  66
     7.4.  Server Responses - Message Status . . . . . . . . . . . .  67
       7.4.1.  EXPUNGE Response  . . . . . . . . . . . . . . . . . .  67
       7.4.2.  FETCH Response  . . . . . . . . . . . . . . . . . . .  68
     7.5.  Server Responses - Command Continuation Request . . . . .  73
   8.  Sample IMAP4rev1 connection . . . . . . . . . . . . . . . . .  73
   9.  Formal Syntax . . . . . . . . . . . . . . . . . . . . . . . .  75
   10. Author's Note . . . . . . . . . . . . . . . . . . . . . . . .  85
   11. Security Considerations . . . . . . . . . . . . . . . . . . .  85
     11.1.  STARTTLS Security Considerations . . . . . . . . . . . .  85
     11.2.  Other Security Considerations  . . . . . . . . . . . . .  86
   12. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  87
   13. References  . . . . . . . . . . . . . . . . . . . . . . . . .  87
     13.1.  Normative References . . . . . . . . . . . . . . . . . .  87
     13.2.  Informative References (related protocols) . . . . . . .  89
     13.3.  Informative References (historical versions of IMAP) . .  90
   Appendix A.  Changes from RFC 3501  . . . . . . . . . . . . . . .  90
   Appendix B.  Acknowledgement  . . . . . . . . . . . . . . . . . .  90
   Appendix C.  Key Word Index . . . . . . . . . . . . . . . . . . .  91
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  91

1.  How to Read This Document

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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

   "Conventions" are basic principles or procedures.  Document
   conventions are noted in this section.

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

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "MAY", and "OPTIONAL" in this document are to
   be interpreted as described in [KEYWORDS].

   The word "can" (not "may") is used to refer 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.

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   There are several protocol conventions in IMAP.  These refer to
   aspects of the specification which are not strictly part of the IMAP
   protocol, but reflect generally-accepted practice.  Implementations
   need to be aware of these conventions, and avoid conflicts whether or
   not they implement the convention.  For example, "&" may not be used
   as a hierarchy delimiter since it conflicts with the Mailbox
   International Naming Convention, and other uses of "&" in mailbox
   names are impacted as well.

1.3.  Special Notes to Implementors

   Implementors of the IMAP protocol are strongly encouraged to read the
   IMAP implementation recommendations document [IMAP-IMPLEMENTATION] in
   conjunction with this document, to help understand the intricacies of
   this protocol and how best to build an interoperable product.

   IMAP4rev1 is designed to be upwards compatible from the [IMAP2] and
   unpublished IMAP2bis protocols.  IMAP4rev1 is largely compatible with
   the IMAP4 protocol described in RFC 1730; the exception being in
   certain facilities added in RFC 1730 that proved problematic and were
   subsequently removed.  In the course of the evolution of IMAP4rev1,
   some aspects in the earlier protocols have become obsolete.  Obsolete
   commands, responses, and data formats which an IMAP4rev1
   implementation can encounter when used with an earlier implementation
   are described in [IMAP-OBSOLETE].

   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.

   IMAP was originally developed for the older [RFC-822] standard, and
   as a consequence several fetch items in IMAP incorporate "RFC822" in
   their name.  With the exception of RFC822.SIZE, there are more modern
   replacements; for example, the modern version of RFC822.HEADER is
   BODY.PEEK[HEADER].  In all cases, "RFC822" should be interpreted as a
   reference to the updated [RFC-2822] standard.

2.  Protocol Overview

2.1.  Link Level

   The IMAP4rev1 protocol assumes a reliable data stream such as that
   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.

   Clients MUST follow the syntax outlined in this specification
   strictly.  It is a syntax error to send a command with missing or
   extraneous spaces or arguments.

   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 a 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.

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   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.

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 a protocol error such as
   unrecognized command or command syntax error).

   Servers SHOULD enforce the syntax outlined in this specification
   strictly.  Any client command with a protocol syntax error, including
   (but not limited to) missing or extraneous spaces or arguments,
   SHOULD be rejected, and the client given a BAD server completion
   response.

   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 can be retrieved individually
   or in conjunction with other attributes or message texts.

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2.3.1.  Message Numbers

   Messages in IMAP4rev1 are accessed by one of two numbers; the unique
   identifier or 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
   that MUST NOT refer to any other message in the mailbox or any
   subsequent mailbox with the same name forever.  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.

   The unique identifier of a message MUST NOT change during the
   session, and SHOULD NOT change between sessions.  Any change of
   unique identifiers between sessions MUST be detectable using the
   UIDVALIDITY mechanism discussed below.  Persistent unique identifiers
   are required for 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 are two values which aid in unique
   identifier handling: the next unique identifier value and the unique
   identifier validity value.

   The next unique identifier value is the predicted value that will be
   assigned to a new message in the mailbox.  Unless the unique
   identifier validity also changes (see below), the next unique
   identifier value MUST have the following two characteristics.  First,
   the next unique identifier value MUST NOT change unless new messages
   are added to the mailbox; and second, the next unique identifier
   value MUST change whenever new messages are added to the mailbox,
   even if those new messages are subsequently expunged.

      Note: The next unique identifier value is intended to provide a
      means for a client to determine whether any messages have been
      delivered to the mailbox since the previous time it checked this
      value.  It is not intended to provide any guarantee that any
      message will have this unique identifier.  A client can only
      assume, at the time that it obtains the next unique identifier
      value, that messages arriving after that time will have a UID
      greater than or equal to that value.

   The unique identifier validity value is sent in a UIDVALIDITY
   response code in an OK untagged response at mailbox selection time.

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   If unique identifiers from an earlier session fail to persist in this
   session, the unique identifier validity value MUST be greater than
   the one used in the earlier session.

      Note: Ideally, unique identifiers SHOULD persist at all times.
      Although this specification recognizes that failure to persist can
      be unavoidable in certain server environments, it STRONGLY
      ENCOURAGES message store implementation techniques that avoid this
      problem.  For example:

      1.  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
          identifiers are no longer strictly ascending as a result of
          the re-ordering.

      2.  If the message store has no mechanism to store unique
          identifiers, it must regenerate unique identifiers at each
          session, and each session must have a unique UIDVALIDITY
          value.

      3.  If the mailbox is deleted and a new mailbox with the same name
          is created at a later date, the server must either keep track
          of unique identifiers from the previous instance of the
          mailbox, or it must assign a new UIDVALIDITY value to the new
          instance of the mailbox.  A good UIDVALIDITY value to use in
          this case 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.

      4.  The combination of mailbox name, UIDVALIDITY, and UID must
          refer to a single immutable message on that server forever.
          In particular, the internal date, [RFC-2822] size, envelope,
          body structure, and message texts (RFC822, RFC822.HEADER,
          RFC822.TEXT, and all BODY[...] fetch data items) must never
          change.  This does not include message numbers, nor does it
          include attributes that can be set by a STORE command (e.g.,
          FLAGS).

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

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   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.  The number of messages in the mailbox is also
   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 "11 EXISTS" 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 can 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).

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

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      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-arrived 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 can 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, concurrent and 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 or APPEND
      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-2822] 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.

2.3.4.  [RFC-2822] Size Message Attribute

   The number of octets in the message, as expressed in [RFC-2822]
   format.

2.3.5.  Envelope Structure Message Attribute

   A parsed representation of the [RFC-2822] header of the message.
   Note that the IMAP Envelope structure is not the same as an [SMTP]
   envelope.

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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-2822] text of a
   message, IMAP4rev1 permits the fetching of portions of the full
   message text.  Specifically, it is possible to fetch the [RFC-2822]
   message header, [RFC-2822] message body, a [MIME-IMB] body part, or a
   [MIME-IMB] header.

3.  State and Flow Diagram

   Once the connection between client and server is established, an
   IMAP4rev1 connection is in one of four states.  The initial state is
   identified in the server greeting.  Most commands are only valid in
   certain states.  It is a protocol error for the client to attempt a
   command while the connection is in an inappropriate state, and the
   server will respond with a BAD or NO (depending upon server
   implementation) command completion result.

3.1.  Not Authenticated State

   In the not 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 the 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, after an error in selecting a mailbox, or after a
   successful CLOSE command.

3.3.  Selected State

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

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3.4.  Logout State

   In the logout state, the connection is being terminated.  This state
   can be entered as a result of a client request (via the LOGOUT
   command) or by unilateral action on the part of either the client or
   server.

   If the client requests the logout state, the server MUST send an
   untagged BYE response and a tagged OK response to the LOGOUT command
   before the server closes the connection; and the client MUST read the
   tagged OK response to the LOGOUT command before the client closes the
   connection.

   A server MUST NOT unilaterally close the connection without sending
   an untagged BYE response that contains the reason for having done so.
   A client SHOULD NOT unilaterally close the connection, and instead
   SHOULD issue a LOGOUT command.  If the server detects that the client
   has unilaterally closed the connection, the server MAY omit the
   untagged BYE response and simply close its connection.

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                      +----------------------+
                      |connection established|
                      +----------------------+
                                 ||
                                 \/
               +--------------------------------------+
               |          server greeting             |
               +--------------------------------------+
                         || (1)       || (2)        || (3)
                         \/           ||            ||
               +-----------------+    ||            ||
               |Not Authenticated|    ||            ||
               +-----------------+    ||            ||
                || (7)   || (4)       ||            ||
                ||       \/           \/            ||
                ||     +----------------+           ||
                ||     | Authenticated  |<=++       ||
                ||     +----------------+  ||       ||
                ||       || (7)   || (5)   || (6)   ||
                ||       ||       \/       ||       ||
                ||       ||    +--------+  ||       ||
                ||       ||    |Selected|==++       ||
                ||       ||    +--------+           ||
                ||       ||       || (7)            ||
                \/       \/       \/                \/
               +--------------------------------------+
               |               Logout                 |
               +--------------------------------------+
                                 ||
                                 \/
                   +-------------------------------+
                   |both sides close the 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.  Note that a particular data item may take more than one

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   form; for example, a data item defined as using "astring" syntax may
   be either an atom or a string.

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: either literal or 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 which may be used.

   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.

   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

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   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 form "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 ().

      Note: NIL is never used for any data item which takes the form of
      an atom.  For example, a mailbox name of "NIL" is a mailbox named
      NIL as opposed to a non-existent mailbox name.  This is because
      mailbox uses "astring" syntax which is an atom or a string.
      Conversely, an addr-name of NIL is a non-existent personal name,
      because addr-name uses "nstring" syntax which is NIL or a string,
      but never an atom.

5.  Operational Considerations

   The following rules are listed here to ensure that all IMAP4rev1
   implementations interoperate properly.

5.1.  Mailbox Naming

   Mailbox names are 7-bit.  Client implementations MUST NOT attempt to
   create 8-bit mailbox names, and SHOULD interpret any 8-bit mailbox
   names returned by LIST or LSUB as UTF-8.  Server implementations
   SHOULD prohibit the creation of 8-bit mailbox names, and SHOULD NOT
   return 8-bit mailbox names in LIST or LSUB.  See section 5.1.3 for
   more information on how to represent non-ASCII mailbox names.

      Note: 8-bit mailbox names were undefined in earlier versions of
      this protocol.  Some sites used a local 8-bit character set to
      represent non-ASCII mailbox names.  Such usage is not
      interoperable, and is now formally deprecated.

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

   In particular, this specification takes no position on case
   sensitivity in non-INBOX mailbox names.  Some server implementations
   are fully case-sensitive; others preserve case of a newly-created
   name but otherwise are case-insensitive; and yet others coerce names
   to a particular case.  Client implementations MUST interact with any
   of these.  If a server implementation interprets non-INBOX mailbox
   names as case-insensitive, it MUST treat names using the
   international naming convention specially as described in section
   5.1.3.

   There are certain client considerations when creating a new mailbox
   name:

   1.  Any character which is one of the atom-specials (see the Formal
       Syntax) will require that the mailbox name be represented as a
       quoted string or literal.

   2.  CTL and other non-graphic characters are difficult to represent
       in a user interface and are best avoided.

   3.  Although the list-wildcard characters ("%" and "*") are valid in
       a mailbox name, it is difficult to use such mailbox names with
       the LIST and LSUB commands due to the conflict with wildcard
       interpretation.

   4.  Usually, a character (determined by the server implementation) is
       reserved to delimit levels of hierarchy.

   5.  Two characters, "#" and "&", have meanings by convention, and
       should be avoided except when used in that convention.

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.

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      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 a mailbox name of
      "#news.comp.mail.misc", and the name "comp.mail.misc" can refer to
      a different object (e.g., a user's private mailbox).

5.1.3.  Mailbox International Naming Convention

   By convention, international mailbox names in IMAP4rev1 are specified
   using a modified version of the UTF-7 encoding described in [UTF-7].
   Modified UTF-7 may also be usable in servers that implement an
   earlier version of this protocol.

   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 and 0x7f-0xff) 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.  There is no implicit shift from BASE64 to US-ASCII, and null
   shifts ("-&" while in BASE64; note that "&-" while in US-ASCII means
   "&") are not permitted.  However, all names start in US-ASCII, and
   MUST end in US-ASCII; that is, a name that ends with a non-ASCII
   ISO-10646 character MUST end with a "-").

   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.

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

   Although modified UTF-7 is a convention, it establishes certain
   requirements on server handling of any mailbox name with an embedded
   "&" character.  In particular, server implementations MUST preserve
   the exact form of the modified BASE64 portion of a modified UTF-7
   name and treat that text as case-sensitive, even if names are
   otherwise case-insensitive or case-folded.

   Server implementations SHOULD verify that any mailbox name with an
   embedded "&" character, used as an argument to CREATE, is: in the
   correctly modified UTF-7 syntax, has no superfluous shifts, and has
   no encoding in modified BASE64 of any printing US-ASCII character
   which can represent itself.  However, client implementations MUST NOT
   depend upon the server doing this, and SHOULD NOT attempt to create a
   mailbox name with an embedded "&" character unless it complies with
   the modified UTF-7 syntax.

   Server implementations which export a mail store that does not follow
   the modified UTF-7 convention MUST convert to modified UTF-7 any
   mailbox name that contains either non-ASCII characters or the "&"
   character.

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

      For example, the string "&Jjo!" is not a valid mailbox name
      because it does not contain a shift to US-ASCII before the "!".
      The correct form is "&Jjo-!".  The string "&U,BTFw-&ZeVnLIqe-" is
      not permitted because it contains a superfluous shift.  The
      correct form is "&U,BTF2XlZyyKng-".

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 message
   delivery), change the flags of the messages 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.

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   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.  In particular,
   it is NOT permitted to send an EXISTS response that would reduce the
   number of messages in the mailbox; only the EXPUNGE response can do
   this.

   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 the
   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, the duration of that
   timer MUST be at least 30 minutes.  The receipt of ANY command from
   the client during that interval SHOULD suffice to reset the
   autologout timer.

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, e.g., a FETCH of a message's flags
   and a STORE of that same message's flags.

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   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 the completion result response before sending a command
   with message sequence numbers.

      Note: UID FETCH, UID STORE, and UID SEARCH are different commands
      from FETCH, STORE, and SEARCH.  If the client sends a UID command,
      it must wait for a completion result 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

      UID SEARCH + UID SEARCH may be valid or invalid as a non-waiting
      command sequence, depending upon whether or not the second UID
      SEARCH contains message sequence numbers.

6.  Client Commands

   IMAP4rev1 commands are described in this section.  Commands are
   organized by the state in which the command is permitted.  Commands
   which are permitted in multiple states are listed in the minimum
   permitted state (for example, commands valid in authenticated and
   selected state are listed in the authenticated state commands).

   Command arguments, identified by "Arguments:" in the command
   descriptions below, are described by function, not by syntax.  The
   precise syntax of command arguments is described in the Formal Syntax
   (Section 9).

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   Some commands cause specific server responses to be returned; these
   are identified by "Responses:" in the command descriptions below.
   See the response descriptions in the Responses section for
   information on these responses, and the Formal Syntax section for the
   precise syntax of these responses.  It is possible for server data to
   be transmitted as a result of any command.  Thus, commands that do
   not specifically require server data specify "no specific responses
   for this command" instead of "none".

   The "Result:" in the command description refers to the possible
   tagged status responses to a command, and any special interpretation
   of these status responses.

   The state of a connection is only changed by successful commands
   which are documented as changing state.  A rejected command (BAD
   response) never changes the state of the connection or of the
   selected mailbox.  A failed command (NO response) generally does not
   change the state of the connection or of the selected mailbox; the
   exception being the SELECT and EXAMINE commands.

6.1.  Client Commands - Any State

   The following commands are valid in any state: CAPABILITY, NOOP, and
   LOGOUT.

6.1.1.  CAPABILITY Command

   Arguments:  none

   Responses:  REQUIRED untagged response: CAPABILITY

   Result:     OK - capability completed
               BAD - command unknown or arguments invalid

   The CAPABILITY command requests a listing of capabilities that the
   server supports.  The server MUST send a single untagged CAPABILITY
   response with "IMAP4rev1" as one of the listed capabilities before
   the (tagged) OK response.

   A capability name which begins with "AUTH=" indicates that the server
   supports that particular authentication mechanism.  All such names
   are, by definition, part of this specification.  For example, the
   authorization capability for an experimental "blurdybloop"
   authenticator would be "AUTH=XBLURDYBLOOP" and not
   "XAUTH=BLURDYBLOOP" or "XAUTH=XBLURDYBLOOP".

   Other capability names refer to extensions, revisions, or amendments
   to this specification.  See the documentation of the CAPABILITY

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   response for additional information.  No capabilities, beyond the
   base IMAP4rev1 set defined in this specification, are enabled without
   explicit client action to invoke the capability.

   Client and server implementations MUST implement the STARTTLS,
   LOGINDISABLED, and AUTH=PLAIN (described in [IMAP-TLS]) capabilities.
   See the Security Considerations section for important information.

   See the section entitled "Client Commands - Experimental/Expansion"
   for information about the form of site or implementation-specific
   capabilities.

   Example:    C: abcd CAPABILITY
               S: * CAPABILITY IMAP4rev1 STARTTLS AUTH=GSSAPI
               LOGINDISABLED
               S: abcd OK CAPABILITY completed
               C: efgh STARTTLS
               S: efgh OK STARTLS completed
               <TLS negotiation, further commands are under [TLS] layer>
               C: ijkl CAPABILITY
               S: * CAPABILITY IMAP4rev1 AUTH=GSSAPI AUTH=PLAIN
               S: ijkl OK CAPABILITY completed

6.1.2.  NOOP Command

   Arguments:  none

   Responses:  no specific responses for this command (but see below)

   Result:     OK - noop completed
               BAD - command unknown or arguments invalid

   The NOOP command always succeeds.  It does nothing.

   Since any command can return a status update as untagged data, the
   NOOP command can be used as a periodic poll for new messages or
   message status updates during a period of inactivity (this is the
   preferred method to do this).  The NOOP command can also be used to
   reset any inactivity autologout timer on the server.

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      Example:    C: a002 NOOP
                  S: a002 OK NOOP completed
                     . . .
                  C: a047 NOOP
                  S: * 22 EXPUNGE
                  S: * 23 EXISTS
                  S: * 3 RECENT
                  S: * 14 FETCH (FLAGS (\Seen \Deleted))
                  S: a047 OK NOOP completed

6.1.3.  LOGOUT Command

   Arguments:  none

   Responses:  REQUIRED untagged response: BYE

   Result:     OK - logout completed
               BAD - command unknown or arguments invalid

   The LOGOUT command informs the server that the client is done with
   the connection.  The server MUST send a BYE untagged response before
   the (tagged) OK response, and then close the network connection.

      Example:    C: A023 LOGOUT
                  S: * BYE IMAP4rev1 Server logging out
                  S: A023 OK LOGOUT completed
                  (Server and client then close the connection)

6.2.  Client Commands - Not Authenticated State

   In the not authenticated state, the AUTHENTICATE or LOGIN command
   establishes authentication and enters the authenticated state.  The
   AUTHENTICATE command provides a general mechanism for a variety of
   authentication techniques, privacy protection, and integrity
   checking; whereas the LOGIN command uses a traditional user name and
   plaintext password pair and has no means of establishing privacy
   protection or integrity checking.

   The STARTTLS command is an alternate form of establishing session
   privacy protection and integrity checking, but does not establish
   authentication or enter the authenticated state.

   Server implementations MAY allow access to certain mailboxes without
   establishing authentication.  This can be done by means of the
   ANONYMOUS [SASL] authenticator described in [ANONYMOUS].  An older
   convention is a LOGIN command using the userid "anonymous"; in this
   case, a password is required although the server may choose to accept

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   any password.  The restrictions placed on anonymous users are
   implementation-dependent.

   Once authenticated (including as anonymous), it is not possible to
   re-enter not authenticated state.

   In addition to the universal commands (CAPABILITY, NOOP, and LOGOUT),
   the following commands are valid in the not authenticated state:
   STARTTLS, AUTHENTICATE and LOGIN.  See the Security Considerations
   section for important information about these commands.

6.2.1.  STARTTLS Command

   Arguments:  none

   Responses:  no specific response for this command

   Result:     OK - starttls completed, begin TLS negotiation
               BAD - command unknown or arguments invalid

   A [TLS] negotiation begins immediately after the CRLF at the end of
   the tagged OK response from the server.  Once a client issues a
   STARTTLS command, it MUST NOT issue further commands until a server
   response is seen and the [TLS] negotiation is complete.

   The server remains in the non-authenticated state, even if client
   credentials are supplied during the [TLS] negotiation.  This does not
   preclude an authentication mechanism such as EXTERNAL (defined in
   [SASL]) from using client identity determined by the [TLS]
   negotiation.

   Once [TLS] has been started, the client MUST discard cached
   information about server capabilities and SHOULD re-issue the
   CAPABILITY command.  This is necessary to protect against man-in-
   the-middle attacks which alter the capabilities list prior to
   STARTTLS.  The server MAY advertise different capabilities after
   STARTTLS.

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   Example:    C: a001 CAPABILITY
               S: * CAPABILITY IMAP4rev1 STARTTLS LOGINDISABLED
               S: a001 OK CAPABILITY completed
               C: a002 STARTTLS
               S: a002 OK Begin TLS negotiation now
               <TLS negotiation, further commands are under [TLS] layer>
               C: a003 CAPABILITY
               S: * CAPABILITY IMAP4rev1 AUTH=PLAIN
               S: a003 OK CAPABILITY completed
               C: a004 LOGIN joe password
               S: a004 OK LOGIN completed

6.2.2.  AUTHENTICATE Command

   Arguments:  authentication mechanism name

   Responses:  continuation data can be requested

   Result:     OK - authenticate completed, now in authenticated state
               NO - authenticate failure: unsupported authentication
               mechanism, credentials rejected
               BAD - command unknown or arguments invalid,
               authentication exchange cancelled

   The AUTHENTICATE command indicates a [SASL] authentication mechanism
   to the server.  If the server supports the requested authentication
   mechanism, it performs an authentication protocol exchange to
   authenticate and identify the client.  It MAY also negotiate an
   OPTIONAL security layer for subsequent protocol interactions.  If the
   requested authentication mechanism is not supported, the server
   SHOULD reject the AUTHENTICATE command by sending a tagged NO
   response.

   The AUTHENTICATE command does not support the optional "initial
   response" feature of [SASL].  Section 5.1 of [SASL] specifies how to
   handle an authentication mechanism which uses an initial response.

   The service name specified by this protocol's profile of [SASL] is
   "imap".

   The authentication protocol exchange consists of a series of server
   challenges and client responses that are specific to the
   authentication mechanism.  A server challenge consists of a command
   continuation request response with the "+" token followed by a BASE64
   encoded string.  The client response consists of a single line
   consisting of a BASE64 encoded string.  If the client wishes to
   cancel an authentication exchange, it issues a line consisting of a

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   single "*".  If the server receives such a response, it MUST reject
   the AUTHENTICATE command by sending a tagged BAD response.

   If a security layer is negotiated through the [SASL] authentication
   exchange, it takes effect immediately following the CRLF that
   concludes the authentication exchange for the client, and the CRLF of
   the tagged OK response for the server.

   While client and server implementations MUST implement the
   AUTHENTICATE command itself, it is not required to implement any
   authentication mechanisms other than the PLAIN mechanism described in
   [IMAP-TLS].  Also, an authentication mechanism is not required to
   support any security layers.

      Note: a server implementation MUST implement a configuration in
      which it does NOT permit any plaintext password mechanisms, unless
      either the STARTTLS command has been negotiated or some other
      mechanism that protects the session from password snooping has
      been provided.  Server sites SHOULD NOT use any configuration
      which permits a plaintext password mechanism without such a
      protection mechanism against password snooping.  Client and server
      implementations SHOULD implement additional [SASL] mechanisms that
      do not use plaintext passwords, such the GSSAPI mechanism
      described in [SASL] and/or the [DIGEST-MD5] mechanism.

   Servers and clients can support multiple authentication mechanisms.
   The server SHOULD list its supported authentication mechanisms in the
   response to the CAPABILITY command so that the client knows which
   authentication mechanisms to use.

   A server MAY include a CAPABILITY response code in the tagged OK
   response of a successful AUTHENTICATE command in order to send
   capabilities automatically.  It is unnecessary for a client to send a
   separate CAPABILITY command if it recognizes these automatic
   capabilities.  This should only be done if a security layer was not
   negotiated by the AUTHENTICATE command, because the tagged OK
   response as part of an AUTHENTICATE command is not protected by
   encryption/integrity checking.  [SASL] requires the client to re-
   issue a CAPABILITY command in this case.

   If an AUTHENTICATE command fails with a NO response, the client MAY
   try another authentication mechanism by issuing another AUTHENTICATE
   command.  It MAY also attempt to authenticate by using the LOGIN
   command (see section 6.2.3 for more detail).  In other words, the
   client MAY request authentication types in decreasing order of
   preference, with the LOGIN command as a last resort.

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   The authorization identity passed from the client to the server
   during the authentication exchange is interpreted by the server as
   the user name whose privileges the client is requesting.

      Example:    S: * OK IMAP4rev1 Server
                  C: A001 AUTHENTICATE GSSAPI
                  S: +
                  C: YIIB+wYJKoZIhvcSAQICAQBuggHqMIIB5qADAgEFoQMCAQ6iBw
                     MFACAAAACjggEmYYIBIjCCAR6gAwIBBaESGxB1Lndhc2hpbmd0
                     b24uZWR1oi0wK6ADAgEDoSQwIhsEaW1hcBsac2hpdmFtcy5jYW
                     Mud2FzaGluZ3Rvbi5lZHWjgdMwgdCgAwIBAaEDAgEDooHDBIHA
                     cS1GSa5b+fXnPZNmXB9SjL8Ollj2SKyb+3S0iXMljen/jNkpJX
                     AleKTz6BQPzj8duz8EtoOuNfKgweViyn/9B9bccy1uuAE2HI0y
                     C/PHXNNU9ZrBziJ8Lm0tTNc98kUpjXnHZhsMcz5Mx2GR6dGknb
                     I0iaGcRerMUsWOuBmKKKRmVMMdR9T3EZdpqsBd7jZCNMWotjhi
                     vd5zovQlFqQ2Wjc2+y46vKP/iXxWIuQJuDiisyXF0Y8+5GTpAL
                     pHDc1/pIGmMIGjoAMCAQGigZsEgZg2on5mSuxoDHEA1w9bcW9n
                     FdFxDKpdrQhVGVRDIzcCMCTzvUboqb5KjY1NJKJsfjRQiBYBdE
                     NKfzK+g5DlV8nrw81uOcP8NOQCLR5XkoMHC0Dr/80ziQzbNqhx
                     O6652Npft0LQwJvenwDI13YxpwOdMXzkWZN/XrEqOWp6GCgXTB
                     vCyLWLlWnbaUkZdEYbKHBPjd8t/1x5Yg==
                  S: + YGgGCSqGSIb3EgECAgIAb1kwV6ADAgEFoQMCAQ+iSzBJoAMC
                     AQGiQgRAtHTEuOP2BXb9sBYFR4SJlDZxmg39IxmRBOhXRKdDA0
                     uHTCOT9Bq3OsUTXUlk0CsFLoa8j+gvGDlgHuqzWHPSQg==
                  C:
                  S: + YDMGCSqGSIb3EgECAgIBAAD/////6jcyG4GE3KkTzBeBiVHe
                     ceP2CWY0SR0fAQAgAAQEBAQ=
                  C: YDMGCSqGSIb3EgECAgIBAAD/////3LQBHXTpFfZgrejpLlLImP
                     wkhbfa2QteAQAgAG1yYwE=
                  S: A001 OK GSSAPI authentication successful

   Note: The line breaks within server challenges and client responses
   are for editorial clarity and are not in real authenticators.

6.2.3.  LOGIN Command

   Arguments:  user name
               password

   Responses:  no specific responses for this command

   Result:     OK - login completed, now in authenticated state
               NO - login failure: user name or password rejected
               BAD - command unknown or arguments invalid

   The LOGIN command identifies the client to the server and carries the
   plaintext password authenticating this user.

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   A server MAY include a CAPABILITY response code in the tagged OK
   response to a successful LOGIN command in order to send capabilities
   automatically.  It is unnecessary for a client to send a separate
   CAPABILITY command if it recognizes these automatic capabilities.

      Example:    C: a001 LOGIN SMITH SESAME
                  S: a001 OK LOGIN completed

   Note: Use of the LOGIN command over an insecure network (such as the
   Internet) is a security risk, because anyone monitoring network
   traffic can obtain plaintext passwords.  The LOGIN command SHOULD NOT
   be used except as a last resort, and it is recommended that client
   implementations have a means to disable any automatic use of the
   LOGIN command.

   Unless either the STARTTLS command has been negotiated or some other
   mechanism that protects the session from password snooping has been
   provided, a server implementation MUST implement a configuration in
   which it advertises the LOGINDISABLED capability and does NOT permit
   the LOGIN command.  Server sites SHOULD NOT use any configuration
   which permits the LOGIN command without such a protection mechanism
   against password snooping.  A client implementation MUST NOT send a
   LOGIN command if the LOGINDISABLED capability is advertised.

6.3.  Client Commands - Authenticated State

   In the authenticated state, commands that manipulate mailboxes as
   atomic entities are permitted.  Of these commands, the SELECT and
   EXAMINE commands will select a mailbox for access and enter the
   selected state.

   In addition to the universal commands (CAPABILITY, NOOP, and LOGOUT),
   the following commands are valid in the authenticated state: SELECT,
   EXAMINE, CREATE, DELETE, RENAME, SUBSCRIBE, UNSUBSCRIBE, LIST, LSUB,
   STATUS, and APPEND.

6.3.1.  SELECT Command

   Arguments:  mailbox name

   Responses:  REQUIRED untagged responses: FLAGS, EXISTS, RECENT
               REQUIRED OK untagged responses: UNSEEN, PERMANENTFLAGS,
               UIDNEXT, UIDVALIDITY

   Result:     OK - select completed, now in selected state
               NO - select failure, now in authenticated state: no
               such mailbox, can't access mailbox
               BAD - command unknown or arguments invalid

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   The SELECT command selects a mailbox so that messages in the mailbox
   can be accessed.  Before returning an OK to the client, the server
   MUST send the following untagged data to the client.  Note that
   earlier versions of this protocol only required the FLAGS, EXISTS,
   and RECENT untagged data; consequently, client implementations SHOULD
   implement default behavior for missing data as discussed with the
   individual item.

   FLAGS  Defined flags in the mailbox.  See the description of the
      FLAGS response for more detail.

   <n> EXISTS  The number of messages in the mailbox.  See the
      description of the EXISTS response for more detail.

   <n> RECENT  The number of messages with the \Recent flag set.  See
      the description of the RECENT response for more detail.

   OK [UNSEEN <n>]  The message sequence number of the first unseen
      message in the mailbox.  If this is missing, the client can not
      make any assumptions about the first unseen message in the
      mailbox, and needs to issue a SEARCH command if it wants to find
      it.

   OK [PERMANENTFLAGS (<list of flags>)]  A list of message flags that
      the client can change permanently.  If this is missing, the client
      should assume that all flags can be changed permanently.

   OK [UIDNEXT <n>]  The next unique identifier value.  Refer to section
      2.3.1.1 for more information.  If this is missing, the client can
      not make any assumptions about the next unique identifier value.

   OK [UIDVALIDITY <n>]  The unique identifier validity value.  Refer to
      section 2.3.1.1 for more information.  If this is missing, the
      server does not support unique identifiers.

   Only one mailbox can be selected at a time in a connection;
   simultaneous access to multiple mailboxes requires multiple
   connections.  The SELECT command automatically deselects any
   currently selected mailbox before attempting the new selection.
   Consequently, if a mailbox is selected and a SELECT command that
   fails is attempted, no mailbox is selected.

   If the client is permitted to modify the mailbox, the server SHOULD
   prefix the text of the tagged OK response with the "[READ-WRITE]"
   response code.

   If the client is not permitted to modify the mailbox but is permitted
   read access, the mailbox is selected as read-only, and the server

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   MUST prefix the text of the tagged OK response to SELECT with the
   "[READ-ONLY]" response code.  Read-only access through SELECT differs
   from the EXAMINE command in that certain read-only mailboxes MAY
   permit the change of permanent state on a per-user (as opposed to
   global) basis.  Netnews messages marked in a server-based .newsrc
   file are an example of such per-user permanent state that can be
   modified with read-only mailboxes.

      Example:    C: A142 SELECT INBOX
                  S: * 172 EXISTS
                  S: * 1 RECENT
                  S: * OK [UNSEEN 12] Message 12 is first unseen
                  S: * OK [UIDVALIDITY 3857529045] UIDs valid
                  S: * OK [UIDNEXT 4392] Predicted next UID
                  S: * FLAGS (\Answered \Flagged \Deleted \Seen \Draft)
                  S: * OK [PERMANENTFLAGS (\Deleted \Seen \*)] Limited
                  S: A142 OK [READ-WRITE] SELECT completed

6.3.2.  EXAMINE Command

   Arguments:  mailbox name

   Responses:  REQUIRED untagged responses: FLAGS, EXISTS, RECENT
               REQUIRED OK untagged responses: UNSEEN, PERMANENTFLAGS,
               UIDNEXT, UIDVALIDITY

   Result:     OK - examine completed, now in selected state
               NO - examine failure, now in authenticated state: no
               such mailbox, can't access mailbox BAD - command unknown
               or arguments invalid

   The EXAMINE command is identical to SELECT and returns the same
   output; however, the selected mailbox is identified as read-only.  No
   changes to the permanent state of the mailbox, including per-user
   state, are permitted; in particular, EXAMINE MUST NOT cause messages
   to lose the \Recent flag.

   The text of the tagged OK response to the EXAMINE command MUST begin
   with the "[READ-ONLY]" response code.

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    Example:    C: A932 EXAMINE blurdybloop
                S: * 17 EXISTS
                S: * 2 RECENT
                S: * OK [UNSEEN 8] Message 8 is first unseen
                S: * OK [UIDVALIDITY 3857529045] UIDs valid
                S: * OK [UIDNEXT 4392] Predicted next UID
                S: * FLAGS (\Answered \Flagged \Deleted \Seen \Draft)
                S: * OK [PERMANENTFLAGS ()] No permanent flags permitted
                S: A932 OK [READ-ONLY] EXAMINE completed

6.3.3.  CREATE Command

   Arguments:  mailbox name

   Responses:  no specific responses for this command

   Result:     OK - create completed
               NO - create failure: can't create mailbox with that name
               BAD - command unknown or arguments invalid

   The CREATE command creates a mailbox with the given name.  An OK
   response is returned only if a new mailbox with that name has been
   created.  It is an error to attempt to create INBOX or a mailbox with
   a name that refers to an extant mailbox.  Any error in creation will
   return a tagged NO response.

   If the mailbox name is suffixed with the server's hierarchy separator
   character (as returned from the server by a LIST command), this is a
   declaration that the client intends to create mailbox names under
   this name in the hierarchy.  Server implementations that do not
   require this declaration MUST ignore the declaration.  In any case,
   the name created is without the trailing hierarchy delimiter.

   If the server's hierarchy separator character appears elsewhere in
   the name, the server SHOULD create any superior hierarchical names
   that are needed for the CREATE command to be successfully completed.
   In other words, an attempt to create "foo/bar/zap" on a server in
   which "/" is the hierarchy separator character SHOULD create foo/ and
   foo/bar/ if they do not already exist.

   If a new mailbox is created with the same name as a mailbox which was
   deleted, its unique identifiers MUST be greater than any unique
   identifiers used in the previous incarnation of the mailbox UNLESS
   the new incarnation has a different unique identifier validity value.
   See the description of the UID command for more detail.

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      Example:    C: A003 CREATE owatagusiam/
                  S: A003 OK CREATE completed
                  C: A004 CREATE owatagusiam/blurdybloop
                  S: A004 OK CREATE completed

      Note: The interpretation of this example depends on whether "/"
      was returned as the hierarchy separator from LIST.  If "/" is the
      hierarchy separator, a new level of hierarchy named "owatagusiam"
      with a member called "blurdybloop" is created.  Otherwise, two
      mailboxes at the same hierarchy level are created.

6.3.4.  DELETE Command

   Arguments:  mailbox name

   Responses:  no specific responses for this command

   Result:     OK - delete completed
               NO - delete failure: can't delete mailbox with that name
               BAD - command unknown or arguments invalid

   The DELETE command permanently removes the mailbox with the given
   name.  A tagged OK response is returned only if the mailbox has been
   deleted.  It is an error to attempt to delete INBOX or a mailbox name
   that does not exist.

   The DELETE command MUST NOT remove inferior hierarchical names.  For
   example, if a mailbox "foo" has an inferior "foo.bar" (assuming "."
   is the hierarchy delimiter character), removing "foo" MUST NOT remove
   "foo.bar".  It is an error to attempt to delete a name that has
   inferior hierarchical names and also has the \Noselect mailbox name
   attribute (see the description of the LIST response for more
   details).

   It is permitted to delete a name that has inferior hierarchical names
   and does not have the \Noselect mailbox name attribute.  In this
   case, all messages in that mailbox are removed, and the name will
   acquire the \Noselect mailbox name attribute.

   The value of the highest-used unique identifier of the deleted
   mailbox MUST be preserved so that a new mailbox created with the same
   name will not reuse the identifiers of the former incarnation, UNLESS
   the new incarnation has a different unique identifier validity value.
   See the description of the UID command for more detail.

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      Examples:   C: A682 LIST "" *
                  S: * LIST () "/" blurdybloop
                  S: * LIST (\Noselect) "/" foo
                  S: * LIST () "/" foo/bar
                  S: A682 OK LIST completed
                  C: A683 DELETE blurdybloop
                  S: A683 OK DELETE completed
                  C: A684 DELETE foo
                  S: A684 NO Name "foo" has inferior hierarchical names
                  C: A685 DELETE foo/bar
                  S: A685 OK DELETE Completed
                  C: A686 LIST "" *
                  S: * LIST (\Noselect) "/" foo
                  S: A686 OK LIST completed
                  C: A687 DELETE foo
                  S: A687 OK DELETE Completed
                  C: A82 LIST "" *
                  S: * LIST () "." blurdybloop
                  S: * LIST () "." foo
                  S: * LIST () "." foo.bar
                  S: A82 OK LIST completed
                  C: A83 DELETE blurdybloop
                  S: A83 OK DELETE completed
                  C: A84 DELETE foo
                  S: A84 OK DELETE Completed
                  C: A85 LIST "" *
                  S: * LIST () "." foo.bar
                  S: A85 OK LIST completed
                  C: A86 LIST "" %
                  S: * LIST (\Noselect) "." foo
                  S: A86 OK LIST completed

6.3.5.  RENAME Command

   Arguments:  existing mailbox name
               new mailbox name

   Responses:  no specific responses for this command

   Result:     OK - rename completed
               NO - rename failure: can't rename mailbox with that name,
               can't rename to mailbox with that name
               BAD - command unknown or arguments invalid

   The RENAME command changes the name of a mailbox.  A tagged OK
   response is returned only if the mailbox has been renamed.  It is an
   error to attempt to rename from a mailbox name that does not exist or

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   to a mailbox name that already exists.  Any error in renaming will
   return a tagged NO response.

   If the name has inferior hierarchical names, then the inferior
   hierarchical names MUST also be renamed.  For example, a rename of
   "foo" to "zap" will rename "foo/bar" (assuming "/" is the hierarchy
   delimiter character) to "zap/bar".

   If the server's hierarchy separator character appears in the name,
   the server SHOULD create any superior hierarchical names that are
   needed for the RENAME command to complete successfully.  In other
   words, an attempt to rename "foo/bar/zap" to baz/rag/zowie on a
   server in which "/" is the hierarchy separator character SHOULD
   create baz/ and baz/rag/ if they do not already exist.

   The value of the highest-used unique identifier of the old mailbox
   name MUST be preserved so that a new mailbox created with the same
   name will not reuse the identifiers of the former incarnation, UNLESS
   the new incarnation has a different unique identifier validity value.
   See the description of the UID command for more detail.

   Renaming INBOX is permitted, and has special behavior.  It moves all
   messages in INBOX to a new mailbox with the given name, leaving INBOX
   empty.  If the server implementation supports inferior hierarchical
   names of INBOX, these are unaffected by a rename of INBOX.

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      Examples:   C: A682 LIST "" *
                  S: * LIST () "/" blurdybloop
                  S: * LIST (\Noselect) "/" foo
                  S: * LIST () "/" foo/bar
                  S: A682 OK LIST completed
                  C: A683 RENAME blurdybloop sarasoop
                  S: A683 OK RENAME completed
                  C: A684 RENAME foo zowie
                  S: A684 OK RENAME Completed
                  C: A685 LIST "" *
                  S: * LIST () "/" sarasoop
                  S: * LIST (\Noselect) "/" zowie
                  S: * LIST () "/" zowie/bar
                  S: A685 OK LIST completed

                  C: Z432 LIST "" *
                  S: * LIST () "." INBOX
                  S: * LIST () "." INBOX.bar
                  S: Z432 OK LIST completed
                  C: Z433 RENAME INBOX old-mail
                  S: Z433 OK RENAME completed
                  C: Z434 LIST "" *
                  S: * LIST () "." INBOX
                  S: * LIST () "." INBOX.bar
                  S: * LIST () "." old-mail
                  S: Z434 OK LIST completed

6.3.6.  SUBSCRIBE Command

   Arguments:  mailbox

   Responses:  no specific responses for this command

   Result:     OK - subscribe completed
               NO - subscribe failure: can't subscribe to that name
               BAD - command unknown or arguments invalid

   The SUBSCRIBE command adds the specified mailbox name to the server's
   set of "active" or "subscribed" mailboxes as returned by the LSUB
   command.  This command returns a tagged OK response only if the
   subscription is successful.

   A server MAY validate the mailbox argument to SUBSCRIBE to verify
   that it exists.  However, it MUST NOT unilaterally remove an existing
   mailbox name from the subscription list even if a mailbox by that
   name no longer exists.

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      Note: This requirement is because a server site can choose to
      routinely remove a mailbox with a well-known name (e.g., "system-
      alerts") after its contents expire, with the intention of
      recreating it when new contents are appropriate.

      Example:    C: A002 SUBSCRIBE #news.comp.mail.mime
                  S: A002 OK SUBSCRIBE completed

6.3.7.  UNSUBSCRIBE Command

   Arguments:  mailbox name

   Responses:  no specific responses for this command

   Result:     OK - unsubscribe completed
               NO - unsubscribe failure: can't unsubscribe that name
               BAD - command unknown or arguments invalid

   The UNSUBSCRIBE command removes the specified mailbox name from the
   server's set of "active" or "subscribed" mailboxes as returned by the
   LSUB command.  This command returns a tagged OK response only if the
   unsubscription is successful.

      Example:    C: A002 UNSUBSCRIBE #news.comp.mail.mime
                  S: A002 OK UNSUBSCRIBE completed

6.3.8.  LIST Command

   Arguments:  reference name
               mailbox name with possible wildcards

   Responses:  untagged responses: LIST

   Result:     OK - list completed
               NO - list failure: can't list that reference or name
               BAD - command unknown or arguments invalid

   The LIST command returns a subset of names from the complete set of
   all names available to the client.  Zero or more untagged LIST
   replies are returned, containing the name attributes, hierarchy
   delimiter, and name; see the description of the LIST reply for more
   detail.

   The LIST command SHOULD return its data quickly, without undue delay.
   For example, it SHOULD NOT go to excess trouble to calculate the
   \Marked or \Unmarked status or perform other processing; if each name
   requires 1 second of processing, then a list of 1200 names would take
   20 minutes!

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   An empty ("" string) reference name argument indicates that the
   mailbox name is interpreted as by SELECT.  The returned mailbox names
   MUST match the supplied mailbox name pattern.  A non-empty reference
   name argument is the name of a mailbox or a level of mailbox
   hierarchy, and indicates the context in which the mailbox name is
   interpreted.

   An empty ("" string) mailbox name argument is a special request to
   return the hierarchy delimiter and the root name of the name given in
   the reference.  The value returned as the root MAY be the empty
   string if the reference is non-rooted or is an empty string.  In all
   cases, a hierarchy delimiter (or NIL if there is no hierarchy) is
   returned.  This permits a client to get the hierarchy delimiter (or
   find out that the mailbox names are flat) even when no mailboxes by
   that name currently exist.

   The reference and mailbox name arguments are interpreted into a
   canonical form that represents an unambiguous left-to-right
   hierarchy.  The returned mailbox names will be in the interpreted
   form.

      Note: The interpretation of the reference argument is
      implementation-defined.  It depends upon whether the server
      implementation has a concept of the "current working directory"
      and leading "break out characters", which override the current
      working directory.

      For example, on a server which exports a UNIX or NT filesystem,
      the reference argument contains the current working directory, and
      the mailbox name argument would contain the name as interpreted in
      the current working directory.

      If a server implementation has no concept of break out characters,
      the canonical form is normally the reference name appended with
      the mailbox name.  Note that if the server implements the
      namespace convention (section 5.1.2), "#" is a break out character
      and must be treated as such.

      If the reference argument is not a level of mailbox hierarchy
      (that is, it is a \NoInferiors name), and/or the reference
      argument does not end with the hierarchy delimiter, it is
      implementation-dependent how this is interpreted.  For example, a
      reference of "foo/bar" and mailbox name of "rag/baz" could be
      interpreted as "foo/bar/rag/baz", "foo/barrag/baz", or "foo/rag/
      baz".  A client SHOULD NOT use such a reference argument except at
      the explicit request of the user.  A hierarchical browser MUST NOT
      make any assumptions about server interpretation of the reference

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      unless the reference is a level of mailbox hierarchy AND ends with
      the hierarchy delimiter.

   Any part of the reference argument that is included in the
   interpreted form SHOULD prefix the interpreted form.  It SHOULD also
   be in the same form as the reference name argument.  This rule
   permits the client to determine if the returned mailbox name is in
   the context of the reference argument, or if something about the
   mailbox argument overrode the reference argument.  Without this rule,
   the client would have to have knowledge of the server's naming
   semantics including what characters are "breakouts" that override a
   naming context.

              For example, here are some examples of how references
              and mailbox names might be interpreted on a UNIX-based
              server:

                  Reference     Mailbox Name  Interpretation
                  ------------  ------------  --------------
                  ~smith/Mail/  foo.*         ~smith/Mail/foo.*
                  archive/      %             archive/%
                  #news.        comp.mail.*   #news.comp.mail.*
                  ~smith/Mail/  /usr/doc/foo  /usr/doc/foo
                  archive/      ~fred/Mail/*  ~fred/Mail/*

              The first three examples demonstrate interpretations in
              the context of the reference argument.  Note that
              "~smith/Mail" SHOULD NOT be transformed into something
              like "/u2/users/smith/Mail", or it would be impossible
              for the client to determine that the interpretation was
              in the context of the reference.

   The character "*" is a wildcard, and matches zero or more characters
   at this position.  The character "%" is similar to "*", but it does
   not match a hierarchy delimiter.  If the "%" wildcard is the last
   character of a mailbox name argument, matching levels of hierarchy
   are also returned.  If these levels of hierarchy are not also
   selectable mailboxes, they are returned with the \Noselect mailbox
   name attribute (see the description of the LIST response for more
   details).

   Server implementations are permitted to "hide" otherwise accessible
   mailboxes from the wildcard characters, by preventing certain
   characters or names from matching a wildcard in certain situations.
   For example, a UNIX-based server might restrict the interpretation of
   "*" so that an initial "/" character does not match.

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   The special name INBOX is included in the output from LIST, if INBOX
   is supported by this server for this user and if the uppercase string
   "INBOX" matches the interpreted reference and mailbox name arguments
   with wildcards as described above.  The criteria for omitting INBOX
   is whether SELECT INBOX will return failure; it is not relevant
   whether the user's real INBOX resides on this or some other server.

      Example:    C: A101 LIST "" ""
                  S: * LIST (\Noselect) "/" ""
                  S: A101 OK LIST Completed
                  C: A102 LIST #news.comp.mail.misc ""
                  S: * LIST (\Noselect) "." #news.
                  S: A102 OK LIST Completed
                  C: A103 LIST /usr/staff/jones ""
                  S: * LIST (\Noselect) "/" /
                  S: A103 OK LIST Completed
                  C: A202 LIST ~/Mail/ %
                  S: * LIST (\Noselect) "/" ~/Mail/foo
                  S: * LIST () "/" ~/Mail/meetings
                  S: A202 OK LIST completed

6.3.9.  LSUB Command

   Arguments:  reference name
               mailbox name with possible wildcards

   Responses:  untagged responses: LSUB

   Result:     OK - lsub completed
               NO - lsub failure: can't list that reference or name
               BAD - command unknown or arguments invalid

   The LSUB command returns a subset of names from the set of names that
   the user has declared as being "active" or "subscribed".  Zero or
   more untagged LSUB replies are returned.  The arguments to LSUB are
   in the same form as those for LIST.

   The returned untagged LSUB response MAY contain different mailbox
   flags from a LIST untagged response.  If this should happen, the
   flags in the untagged LIST are considered more authoritative.

   A special situation occurs when using LSUB with the % wildcard.
   Consider what happens if "foo/bar" (with a hierarchy delimiter of
   "/") is subscribed but "foo" is not.  A "%" wildcard to LSUB must
   return foo, not foo/bar, in the LSUB response, and it MUST be flagged
   with the \Noselect attribute.

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   The server MUST NOT unilaterally remove an existing mailbox name from
   the subscription list even if a mailbox by that name no longer
   exists.

      Example:    C: A002 LSUB "#news." "comp.mail.*"
                  S: * LSUB () "." #news.comp.mail.mime
                  S: * LSUB () "." #news.comp.mail.misc
                  S: A002 OK LSUB completed
                  C: A003 LSUB "#news." "comp.%"
                  S: * LSUB (\NoSelect) "." #news.comp.mail
                  S: A003 OK LSUB completed

6.3.10.  STATUS Command

   Arguments:  mailbox name
               status data item names

   Responses:  untagged responses: STATUS

   Result:     OK - status completed
               NO - status failure: no status for that name
               BAD - command unknown or arguments invalid

   The STATUS command requests the status of the indicated mailbox.  It
   does not change the currently selected mailbox, nor does it affect
   the state of any messages in the queried mailbox (in particular,
   STATUS MUST NOT cause messages to lose the \Recent flag).

   The STATUS command provides an alternative to opening a second
   IMAP4rev1 connection and doing an EXAMINE command on a mailbox to
   query that mailbox's status without deselecting the current mailbox
   in the first IMAP4rev1 connection.

   Unlike the LIST command, the STATUS command is not guaranteed to be
   fast in its response.  Under certain circumstances, it can be quite
   slow.  In some implementations, the server is obliged to open the
   mailbox read-only internally to obtain certain status information.
   Also unlike the LIST command, the STATUS command does not accept
   wildcards.

      Note: The STATUS command is intended to access the status of
      mailboxes other than the currently selected mailbox.  Because the
      STATUS command can cause the mailbox to be opened internally, and
      because this information is available by other means on the
      selected mailbox, the STATUS command SHOULD NOT be used on the
      currently selected mailbox.

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      The STATUS command MUST NOT be used as a "check for new messages
      in the selected mailbox" operation (refer to sections 7, 7.3.1,
      and 7.3.2 for more information about the proper method for new
      message checking).

      Because the STATUS command is not guaranteed to be fast in its
      results, clients SHOULD NOT expect to be able to issue many
      consecutive STATUS commands and obtain reasonable performance.

   The currently defined status data items that can be requested are:

   MESSAGES  The number of messages in the mailbox.

   RECENT  The number of messages with the \Recent flag set.

   UIDNEXT  The next unique identifier value of the mailbox.  Refer to
      section 2.3.1.1 for more information.

   UIDVALIDITY  The unique identifier validity value of the mailbox.
      Refer to section 2.3.1.1 for more information.

   UNSEEN  The number of messages which do not have the \Seen flag set.

      Example:    C: A042 STATUS blurdybloop (UIDNEXT MESSAGES)
                  S: * STATUS blurdybloop (MESSAGES 231 UIDNEXT 44292)
                  S: A042 OK STATUS completed

6.3.11.  APPEND Command

   Arguments:  mailbox name
               OPTIONAL flag parenthesized list
               OPTIONAL date/time string
               message literal

   Responses:  no specific responses for this command

   Result:     OK - append completed
               NO - append error: can't append to that mailbox, error
               in flags or date/time or message text
               BAD - command unknown or arguments invalid

   The APPEND command appends the literal argument as a new message to
   the end of the specified destination mailbox.  This argument SHOULD
   be in the format of an [RFC-2822] message.  8-bit characters are
   permitted in the message.  A server implementation that is unable to
   preserve 8-bit data properly MUST be able to reversibly convert 8-bit
   APPEND data to 7-bit using a [MIME-IMB] content transfer encoding.

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      Note: There MAY be exceptions, e.g., draft messages, in which
      required [RFC-2822] header lines are omitted in the message
      literal argument to APPEND.  The full implications of doing so
      MUST be understood and carefully weighed.

   If a flag parenthesized list is specified, the flags SHOULD be set in
   the resulting message; otherwise, the flag list of the resulting
   message is set to empty by default.  In either case, the Recent flag
   is also set.

   If a date-time is specified, the internal date SHOULD be set in the
   resulting message; otherwise, the internal date of the resulting
   message is set to the current date and time by default.

   If the append is unsuccessful for any reason, the mailbox MUST be
   restored to its state before the APPEND attempt; no partial appending
   is permitted.

   If the destination mailbox does not exist, a server MUST return an
   error, and MUST NOT automatically create the mailbox.  Unless it is
   certain that the destination mailbox can not be created, the server
   MUST send the response code "[TRYCREATE]" as the prefix of the text
   of the tagged NO response.  This gives a hint to the client that it
   can attempt a CREATE command and retry the APPEND if the CREATE is
   successful.

   If the mailbox is currently selected, the normal new message actions
   SHOULD occur.  Specifically, the server SHOULD notify the client
   immediately via an untagged EXISTS response.  If the server does not
   do so, the client MAY issue a NOOP command (or failing that, a CHECK
   command) after one or more APPEND commands.

    Example:    C: A003 APPEND saved-messages (\Seen) {310}
                S: + Ready for literal data
                C: Date: Mon, 7 Feb 1994 21:52:25 -0800 (PST)
                C: From: Fred Foobar <foobar@Blurdybloop.COM>
                C: Subject: afternoon meeting
                C: To: mooch@owatagu.siam.edu
                C: Message-Id: <B27397-0100000@Blurdybloop.COM>
                C: MIME-Version: 1.0
                C: Content-Type: TEXT/PLAIN; CHARSET=US-ASCII
                C:
                C: Hello Joe, do you think we can meet at 3:30 tomorrow?
                C:
                S: A003 OK APPEND completed

   Note: The APPEND command is not used for message delivery, because it
   does not provide a mechanism to transfer [SMTP] envelope information.

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6.4.  Client Commands - Selected State

   In the selected state, commands that manipulate messages in a mailbox
   are permitted.

   In addition to the universal commands (CAPABILITY, NOOP, and LOGOUT),
   and the authenticated state commands (SELECT, EXAMINE, CREATE,
   DELETE, RENAME, SUBSCRIBE, UNSUBSCRIBE, LIST, LSUB, STATUS, and
   APPEND), the following commands are valid in the selected state:
   CHECK, CLOSE, EXPUNGE, SEARCH, FETCH, STORE, COPY, and UID.

6.4.1.  CHECK Command

   Arguments:  none

   Responses:  no specific responses for this command

   Result:     OK - check completed
               BAD - command unknown or arguments invalid

   The CHECK command requests a checkpoint of the currently selected
   mailbox.  A checkpoint refers to any implementation-dependent
   housekeeping associated with the mailbox (e.g., resolving the
   server's in-memory state of the mailbox with the state on its disk)
   that is not normally executed as part of each command.  A checkpoint
   MAY take a non-instantaneous amount of real time to complete.  If a
   server implementation has no such housekeeping considerations, CHECK
   is equivalent to NOOP.

   There is no guarantee that an EXISTS untagged response will happen as
   a result of CHECK.  NOOP, not CHECK, SHOULD be used for new message
   polling.

      Example:    C: FXXZ CHECK
                  S: FXXZ OK CHECK Completed

6.4.2.  CLOSE Command

   Arguments:  none

   Responses:  no specific responses for this command

   Result:     OK - close completed, now in authenticated state
               BAD - command unknown or arguments invalid

   The CLOSE command permanently removes all messages that have the
   \Deleted flag set from the currently selected mailbox, and returns to

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   the authenticated state from the selected state.  No untagged EXPUNGE
   responses are sent.

   No messages are removed, and no error is given, if the mailbox is
   selected by an EXAMINE command or is otherwise selected read-only.

   Even if a mailbox is selected, a SELECT, EXAMINE, or LOGOUT command
   MAY be issued without previously issuing a CLOSE command.  The
   SELECT, EXAMINE, and LOGOUT commands implicitly close the currently
   selected mailbox without doing an expunge.  However, when many
   messages are deleted, a CLOSE-LOGOUT or CLOSE-SELECT sequence is
   considerably faster than an EXPUNGE-LOGOUT or EXPUNGE-SELECT because
   no untagged EXPUNGE responses (which the client would probably
   ignore) are sent.

      Example:    C: A341 CLOSE
                  S: A341 OK CLOSE completed

6.4.3.  EXPUNGE Command

   Arguments:  none

   Responses:  untagged responses: EXPUNGE

   Result:     OK - expunge completed
               NO - expunge failure: can't expunge (e.g., permission
               denied)
               BAD - command unknown or arguments invalid

   The EXPUNGE command permanently removes all messages that have the
   \Deleted flag set from the currently selected mailbox.  Before
   returning an OK to the client, an untagged EXPUNGE response is sent
   for each message that is removed.

      Example:    C: A202 EXPUNGE
                  S: * 3 EXPUNGE
                  S: * 3 EXPUNGE
                  S: * 5 EXPUNGE
                  S: * 8 EXPUNGE
                  S: A202 OK EXPUNGE completed

   Note: In this example, messages 3, 4, 7, and 11 had the \Deleted flag
   set.  See the description of the EXPUNGE response for further
   explanation.

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6.4.4.  SEARCH Command

   Arguments:  OPTIONAL [CHARSET] specification
               searching criteria (one or more)

   Responses:  REQUIRED untagged response: SEARCH

   Result:     OK - search completed
               NO - search error: can't search that [CHARSET] or
               criteria
               BAD - command unknown or arguments invalid

   The SEARCH command searches the mailbox for messages that match the
   given searching criteria.  Searching criteria consist of one or more
   search keys.  The untagged SEARCH response from the server contains a
   listing of message sequence numbers corresponding to those messages
   that match the searching criteria.

   When multiple keys are specified, the result is the intersection (AND
   function) of all the messages that match those keys.  For example,
   the criteria DELETED FROM "SMITH" SINCE 1-Feb-1994 refers to all
   deleted messages from Smith that were placed in the mailbox since
   February 1, 1994.  A search key can also be a parenthesized list of
   one or more search keys (e.g., for use with the OR and NOT keys).

   Server implementations MAY exclude [MIME-IMB] body parts with
   terminal content media types other than TEXT and MESSAGE from
   consideration in SEARCH matching.

   The OPTIONAL [CHARSET] specification consists of the word "CHARSET"
   followed by a registered [CHARSET].  It indicates the [CHARSET] of
   the strings that appear in the search criteria.  [MIME-IMB] content
   transfer encodings, and [MIME-HDRS] strings in [RFC-2822]/[MIME-IMB]
   headers, MUST be decoded before comparing text in a [CHARSET] other
   than US-ASCII.  US-ASCII MUST be supported; other [CHARSET]s MAY be
   supported.

   If the server does not support the specified [CHARSET], it MUST
   return a tagged NO response (not a BAD).  This response SHOULD
   contain the BADCHARSET response code, which MAY list the [CHARSET]s
   supported by the server.

   In all search keys that use strings, a message matches the key if the
   string is a substring of the field.  The matching is case-
   insensitive.

   The defined search keys are as follows.  Refer to the Formal Syntax
   section for the precise syntactic definitions of the arguments.

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   <sequence set>  Messages with message sequence numbers corresponding
      to the specified message sequence number set.

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

   ANSWERED  Messages with the \Answered flag set.

   BCC <string>  Messages that contain the specified string in the
      envelope structure's BCC field.

   BEFORE <date>  Messages whose internal date (disregarding time and
      timezone) is earlier than the specified date.

   BODY <string>  Messages that contain the specified string in the body
      of the message.

   CC <string>  Messages that contain the specified string in the
      envelope structure's CC field.

   DELETED  Messages with the \Deleted flag set.

   DRAFT  Messages with the \Draft flag set.

   FLAGGED  Messages with the \Flagged flag set.

   FROM <string>  Messages that contain the specified string in the
      envelope structure's FROM field.

   HEADER <field-name> <string>  Messages that have a header with the
      specified field-name (as defined in [RFC-2822]) and that contains
      the specified string in the text of the header (what comes after
      the colon).  If the string to search is zero-length, this matches
      all messages that have a header line with the specified field-name
      regardless of the contents.

   KEYWORD <flag>  Messages with the specified keyword flag set.

   LARGER <n>  Messages with an [RFC-2822] size larger than the
      specified number of octets.

   NEW  Messages that have the \Recent flag set but not the \Seen flag.
      This is functionally equivalent to "(RECENT UNSEEN)".

   NOT <search-key>  Messages that do not match the specified search
      key.

   OLD  Messages that do not have the \Recent flag set.  This is
      functionally equivalent to "NOT RECENT" (as opposed to "NOT NEW").

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   ON <date>  Messages whose internal date (disregarding time and
      timezone) is within the specified date.

   OR <search-key1> <search-key2>  Messages that match either search
      key.

   RECENT  Messages that have the \Recent flag set.

   SEEN  Messages that have the \Seen flag set.

   SENTBEFORE <date>  Messages whose [RFC-2822] Date: header
      (disregarding time and timezone) is earlier than the specified
      date.

   SENTON <date>  Messages whose [RFC-2822] Date: header (disregarding
      time and timezone) is within the specified date.

   SENTSINCE <date>  Messages whose [RFC-2822] Date: header
      (disregarding time and timezone) is within or later than the
      specified date.

   SINCE <date>  Messages whose internal date (disregarding time and
      timezone) is within or later than the specified date.

   SMALLER <n>  Messages with an [RFC-2822] size smaller than the
      specified number of octets.

   SUBJECT <string>  Messages that contain the specified string in the
      envelope structure's SUBJECT field.

   TEXT <string>  Messages that contain the specified string in the
      header or body of the message.

   TO <string>  Messages that contain the specified string in the
      envelope structure's TO field.

   UID <sequence set>  Messages with unique identifiers corresponding to
      the specified unique identifier set.  Sequence set ranges are
      permitted.

   UNANSWERED  Messages that do not have the \Answered flag set.

   UNDELETED  Messages that do not have the \Deleted flag set.

   UNDRAFT  Messages that do not have the \Draft flag set.

   UNFLAGGED  Messages that do not have the \Flagged flag set.

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   UNKEYWORD <flag>  Messages that do not have the specified keyword
      flag set.

   UNSEEN  Messages that do not have the \Seen flag set.

    Example:    C: A282 SEARCH FLAGGED SINCE 1-Feb-1994 NOT FROM "Smith"
                S: * SEARCH 2 84 882
                S: A282 OK SEARCH completed
                C: A283 SEARCH TEXT "string not in mailbox"
                S: * SEARCH
                S: A283 OK SEARCH completed
                C: A284 SEARCH CHARSET UTF-8 TEXT {6}
                C: XXXXXX
                S: * SEARCH 43
                S: A284 OK SEARCH completed

   Note: Since this document is restricted to 7-bit ASCII text, it is
   not possible to show actual UTF-8 data.  The "XXXXXX" is a
   placeholder for what would be 6 octets of 8-bit data in an actual
   transaction.

6.4.5.  FETCH Command

   Arguments:  sequence set
               message data item names or macro

   Responses:  untagged responses: FETCH

   Result:     OK - fetch completed
               NO - fetch error: can't fetch that data
               BAD - command unknown or arguments invalid

   The FETCH command retrieves data associated with a message in the
   mailbox.  The data items to be fetched can be either a single atom or
   a parenthesized list.

   Most data items, identified in the formal syntax under the msg-att-
   static rule, are static and MUST NOT change for any particular
   message.  Other data items, identified in the formal syntax under the
   msg-att-dynamic rule, MAY change, either as a result of a STORE
   command or due to external events.

      For example, if a client receives an ENVELOPE for a message when
      it already knows the envelope, it can safely ignore the newly
      transmitted envelope.

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   There are three macros which specify commonly-used sets of data
   items, and can be used instead of data items.  A macro must be used
   by itself, and not in conjunction with other macros or data items.

   ALL  Macro equivalent to: (FLAGS INTERNALDATE RFC822.SIZE ENVELOPE)

   FAST  Macro equivalent to: (FLAGS INTERNALDATE RFC822.SIZE)

   FULL  Macro equivalent to: (FLAGS INTERNALDATE RFC822.SIZE ENVELOPE
      BODY)

   The currently defined data items that can be fetched are:

   BODY  Non-extensible form of BODYSTRUCTURE.

   BODY[<section>]<<partial>>

         The text of a particular body section.  The section
         specification is a set of zero or more part specifiers
         delimited by periods.  A part specifier is either a part number
         or one of the following: HEADER, HEADER.FIELDS,
         HEADER.FIELDS.NOT, MIME, and TEXT.  An empty section
         specification refers to the entire message, including the
         header.

         Every message has at least one part number.  Non-[MIME-IMB]
         messages, and non-multipart [MIME-IMB] messages with no
         encapsulated message, only have a part 1.

         Multipart messages are assigned consecutive part numbers, as
         they occur in the message.  If a particular part is of type
         message or multipart, its parts MUST be indicated by a period
         followed by the part number within that nested multipart part.

         A part of type MESSAGE/RFC822 also has nested part numbers,
         referring to parts of the MESSAGE part's body.

         The HEADER, HEADER.FIELDS, HEADER.FIELDS.NOT, and TEXT part
         specifiers can be the sole part specifier or can be prefixed by
         one or more numeric part specifiers, provided that the numeric
         part specifier refers to a part of type MESSAGE/RFC822.  The
         MIME part specifier MUST be prefixed by one or more numeric
         part specifiers.

         The HEADER, HEADER.FIELDS, and HEADER.FIELDS.NOT part
         specifiers refer to the [RFC-2822] header of the message or of
         an encapsulated [MIME-IMT] MESSAGE/RFC822 message.
         HEADER.FIELDS and HEADER.FIELDS.NOT are followed by a list of

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         field-name (as defined in [RFC-2822]) names, and return a
         subset of the header.  The subset returned by HEADER.FIELDS
         contains only those header fields with a field-name that
         matches one of the names in the list; similarly, the subset
         returned by HEADER.FIELDS.NOT contains only the header fields
         with a non-matching field-name.  The field-matching is case-
         insensitive but otherwise exact.  Subsetting does not exclude
         the [RFC-2822] delimiting blank line between the header and the
         body; the blank line is included in all header fetches, except
         in the case of a message which has no body and no blank line.

         The MIME part specifier refers to the [MIME-IMB] header for
         this part.

         The TEXT part specifier refers to the text body of the message,
         omitting the [RFC-2822] header.

            Here is an example of a complex message with some of its
            part specifiers:

        HEADER     ([RFC-2822] header of the message)
        TEXT       ([RFC-2822] text body of the message) MULTIPART/MIXED
        1          TEXT/PLAIN
        2          APPLICATION/OCTET-STREAM
        3          MESSAGE/RFC822
        3.HEADER   ([RFC-2822] header of the message)
        3.TEXT     ([RFC-2822] text body of the message) MULTIPART/MIXED
        3.1        TEXT/PLAIN
        3.2        APPLICATION/OCTET-STREAM
        4          MULTIPART/MIXED
        4.1        IMAGE/GIF
        4.1.MIME   ([MIME-IMB] header for the IMAGE/GIF)
        4.2        MESSAGE/RFC822
        4.2.HEADER ([RFC-2822] header of the message)
        4.2.TEXT   ([RFC-2822] text body of the message) MULTIPART/MIXED
        4.2.1      TEXT/PLAIN
        4.2.2      MULTIPART/ALTERNATIVE
        4.2.2.1    TEXT/PLAIN
        4.2.2.2    TEXT/RICHTEXT

         It is possible to fetch a substring of the designated text.
         This is done by appending an open angle bracket ("<"), the
         octet position of the first desired octet, a period, the
         maximum number of octets desired, and a close angle bracket
         (">") to the part specifier.  If the starting octet is beyond
         the end of the text, an empty string is returned.

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         Any partial fetch that attempts to read beyond the end of the
         text is truncated as appropriate.  A partial fetch that starts
         at octet 0 is returned as a partial fetch, even if this
         truncation happened.

            Note: This means that BODY[]<0.2048> of a 1500-octet message
            will return BODY[]<0> with a literal of size 1500, not
            BODY[].

            Note: A substring fetch of a HEADER.FIELDS or
            HEADER.FIELDS.NOT part specifier is calculated after
            subsetting the header.

         The \Seen flag is implicitly set; if this causes the flags to
         change, they SHOULD be included as part of the FETCH responses.

   BODY.PEEK[<section>]<<partial>>  An alternate form of BODY[<section>]
      that does not implicitly set the \Seen flag.

   BODYSTRUCTURE  The [MIME-IMB] body structure of the message.  This is
      computed by the server by parsing the [MIME-IMB] header fields in
      the [RFC-2822] header and [MIME-IMB] headers.

   ENVELOPE  The envelope structure of the message.  This is computed by
      the server by parsing the [RFC-2822] header into the component
      parts, defaulting various fields as necessary.

   FLAGS  The flags that are set for this message.

   INTERNALDATE  The internal date of the message.

   RFC822  Functionally equivalent to BODY[], differing in the syntax of
      the resulting untagged FETCH data (RFC822 is returned).

   RFC822.HEADER  Functionally equivalent to BODY.PEEK[HEADER],
      differing in the syntax of the resulting untagged FETCH data
      (RFC822.HEADER is returned).

   RFC822.SIZE  The [RFC-2822] size of the message.

   RFC822.TEXT  Functionally equivalent to BODY[TEXT], differing in the
      syntax of the resulting untagged FETCH data (RFC822.TEXT is
      returned).

   UID  The unique identifier for the message.

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   Example:    C: A654 FETCH 2:4 (FLAGS BODY[HEADER.FIELDS (DATE FROM)])
               S: * 2 FETCH ....
               S: * 3 FETCH ....
               S: * 4 FETCH ....
               S: A654 OK FETCH completed

6.4.6.  STORE Command

   Arguments:  sequence set
               message data item name
               value for message data item

   Responses:  untagged responses: FETCH

   Result:     OK - store completed
               NO - store error: can't store that data
               BAD - command unknown or arguments invalid

   The STORE command alters data associated with a message in the
   mailbox.  Normally, STORE will return the updated value of the data
   with an untagged FETCH response.  A suffix of ".SILENT" in the data
   item name prevents the untagged FETCH, and the server SHOULD assume
   that the client has determined the updated value itself or does not
   care about the updated value.

      Note: Regardless of whether or not the ".SILENT" suffix was used,
      the server SHOULD send an untagged FETCH response if a change to a
      message's flags from an external source is observed.  The intent
      is that the status of the flags is determinate without a race
      condition.

   The currently defined data items that can be stored are:

   FLAGS <flag list>  Replace the flags for the message (other than
      \Recent) with the argument.  The new value of the flags is
      returned as if a FETCH of those flags was done.

   FLAGS.SILENT <flag list>  Equivalent to FLAGS, but without returning
      a new value.

   +FLAGS <flag list>  Add the argument to the flags for the message.
      The new value of the flags is returned as if a FETCH of those
      flags was done.

   +FLAGS.SILENT <flag list>  Equivalent to +FLAGS, but without
      returning a new value.

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   -FLAGS <flag list>  Remove the argument from the flags for the
      message.  The new value of the flags is returned as if a FETCH of
      those flags was done.

   -FLAGS.SILENT <flag list>  Equivalent to -FLAGS, but without
      returning a new value.

      Example:    C: A003 STORE 2:4 +FLAGS (\Deleted)
                  S: * 2 FETCH (FLAGS (\Deleted \Seen))
                  S: * 3 FETCH (FLAGS (\Deleted))
                  S: * 4 FETCH (FLAGS (\Deleted \Flagged \Seen))
                  S: A003 OK STORE completed

6.4.7.  COPY Command

   Arguments:  sequence set
               mailbox name

   Responses:  no specific responses for this command

   Result:     OK - copy completed
               NO - copy error: can't copy those messages or to that
               name
               BAD - command unknown or arguments invalid

   The COPY command copies the specified message(s) to the end of the
   specified destination mailbox.  The flags and internal date of the
   message(s) SHOULD be preserved, and the Recent flag SHOULD be set, in
   the copy.

   If the destination mailbox does not exist, a server SHOULD return an
   error.  It SHOULD NOT automatically create the mailbox.  Unless it is
   certain that the destination mailbox can not be created, the server
   MUST send the response code "[TRYCREATE]" as the prefix of the text
   of the tagged NO response.  This gives a hint to the client that it
   can attempt a CREATE command and retry the COPY if the CREATE is
   successful.

   If the COPY command is unsuccessful for any reason, server
   implementations MUST restore the destination mailbox to its state
   before the COPY attempt.

      Example:    C: A003 COPY 2:4 MEETING
                  S: A003 OK COPY completed

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6.4.8.  UID Command

   Arguments:  command name
               command arguments

   Responses:  untagged responses: FETCH, SEARCH

   Result:     OK - UID command completed
               NO - UID command error
               BAD - command unknown or arguments invalid

   The UID command has two forms.  In the first form, it takes as its
   arguments a COPY, FETCH, or STORE command with arguments appropriate
   for the associated command.  However, the numbers in the sequence set
   argument are unique identifiers instead of message sequence numbers.
   Sequence set ranges are permitted, but there is no guarantee that
   unique identifiers will be contiguous.

   A non-existent unique identifier is ignored without any error message
   generated.  Thus, it is possible for a UID FETCH command to return an
   OK without any data or a UID COPY or UID STORE to return an OK
   without performing any operations.

   In the second form, the UID command takes a SEARCH command with
   SEARCH command arguments.  The interpretation of the arguments is the
   same as with SEARCH; however, the numbers returned in a SEARCH
   response for a UID SEARCH command are unique identifiers instead of
   message sequence numbers.  For example, the command UID SEARCH 1:100
   UID 443:557 returns the unique identifiers corresponding to the
   intersection of two sequence sets, the message sequence number range
   1:100 and the UID range 443:557.

      Note: in the above example, the UID range 443:557 appears.  The
      same comment about a non-existent unique identifier being ignored
      without any error message also applies here.  Hence, even if
      neither UID 443 or 557 exist, this range is valid and would
      include an existing UID 495.

      Also note that a UID range of 559:* always includes the UID of the
      last message in the mailbox, even if 559 is higher than any
      assigned UID value.  This is because the contents of a range are
      independent of the order of the range endpoints.  Thus, any UID
      range with * as one of the endpoints indicates at least one
      message (the message with the highest numbered UID), unless the
      mailbox is empty.

   The number after the "*" in an untagged FETCH response is always a
   message sequence number, not a unique identifier, even for a UID

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   command response.  However, server implementations MUST implicitly
   include the UID message data item as part of any FETCH response
   caused by a UID command, regardless of whether a UID was specified as
   a message data item to the FETCH.

   Note: The rule about including the UID message data item as part of a
   FETCH response primarily applies to the UID FETCH and UID STORE
   commands, including a UID FETCH command that does not include UID as
   a message data item.  Although it is unlikely that the other UID
   commands will cause an untagged FETCH, this rule applies to these
   commands as well.

      Example:    C: A999 UID FETCH 4827313:4828442 FLAGS
                  S: * 23 FETCH (FLAGS (\Seen) UID 4827313)
                  S: * 24 FETCH (FLAGS (\Seen) UID 4827943)
                  S: * 25 FETCH (FLAGS (\Seen) UID 4828442)
                  S: A999 OK UID FETCH completed

6.5.  Client Commands - Experimental/Expansion

6.5.1.  X<atom> Command

   Arguments:  implementation defined

   Responses:  implementation defined

   Result:     OK - command completed
               NO - failure
               BAD - command unknown or arguments invalid

   Any command prefixed with an X is an experimental command.  Commands
   which are not part of this specification, a standard or standards-
   track revision of this specification, or an IESG-approved
   experimental protocol, MUST use the X prefix.

   Any added untagged responses issued by an experimental command MUST
   also be prefixed with an X.  Server implementations MUST NOT send any
   such untagged responses, unless the client requested it by issuing
   the associated experimental command.

      Example:    C: a441 CAPABILITY
                  S: * CAPABILITY IMAP4rev1 XPIG-LATIN
                  S: a441 OK CAPABILITY completed
                  C: A442 XPIG-LATIN
                  S: * XPIG-LATIN ow-nay eaking-spay ig-pay atin-lay
                  S: A442 OK XPIG-LATIN ompleted-cay

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7.  Server Responses

   Server responses are in three forms: status responses, server data,
   and command continuation request.  The information contained in a
   server response, identified by "Contents:" in the response
   descriptions below, is described by function, not by syntax.  The
   precise syntax of server responses is described in the Formal Syntax
   section.

   The client MUST be prepared to accept any response at all times.

   Status responses can be tagged or untagged.  Tagged status responses
   indicate the completion result (OK, NO, or BAD status) of a client
   command, and have a tag matching the command.

   Some status responses, and all server data, are untagged.  An
   untagged response is indicated by the token "*" instead of a tag.
   Untagged status responses indicate server greeting, or server status
   that does not indicate the completion of a command (for example, an
   impending system shutdown alert).  For historical reasons, untagged
   server data responses are also called "unsolicited data", although
   strictly speaking, only unilateral server data is truly
   "unsolicited".

   Certain server data MUST be recorded by the client when it is
   received; this is noted in the description of that data.  Such data
   conveys critical information which affects the interpretation of all
   subsequent commands and responses (e.g., updates reflecting the
   creation or destruction of messages).

   Other server data SHOULD be recorded for later reference; if the
   client does not need to record the data, or if recording the data has
   no obvious purpose (e.g., a SEARCH response when no SEARCH command is
   in progress), the data SHOULD be ignored.

   An example of unilateral untagged server data occurs when the IMAP
   connection is in the selected state.  In the selected state, the
   server checks the mailbox for new messages as part of command
   execution.  Normally, this is part of the execution of every command;
   hence, a NOOP command suffices to check for new messages.  If new
   messages are found, the server sends untagged EXISTS and RECENT
   responses reflecting the new size of the mailbox.  Server
   implementations that offer multiple simultaneous access to the same
   mailbox SHOULD also send appropriate unilateral untagged FETCH and
   EXPUNGE responses if another agent changes the state of any message
   flags or expunges any messages.

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   Command continuation request responses use the token "+" instead of a
   tag.  These responses are sent by the server to indicate acceptance
   of an incomplete client command and readiness for the remainder of
   the command.

7.1.  Server Responses - Status Responses

   Status responses are OK, NO, BAD, PREAUTH and BYE.  OK, NO, and BAD
   can be tagged or untagged.  PREAUTH and BYE are always untagged.

   Status responses MAY include an OPTIONAL "response code".  A response
   code consists of data inside square brackets in the form of an atom,
   possibly followed by a space and arguments.  The response code
   contains additional information or status codes for client software
   beyond the OK/NO/BAD condition, and are defined when there is a
   specific action that a client can take based upon the additional
   information.

   The currently defined response codes are:

   ALERT  The human-readable text contains a special alert that MUST be
      presented to the user in a fashion that calls the user's attention
      to the message.

   BADCHARSET  Optionally followed by a parenthesized list of charsets.
      A SEARCH failed because the given charset is not supported by this
      implementation.  If the optional list of charsets is given, this
      lists the charsets that are supported by this implementation.

   CAPABILITY  Followed by a list of capabilities.  This can appear in
      the initial OK or PREAUTH response to transmit an initial
      capabilities list.  This makes it unnecessary for a client to send
      a separate CAPABILITY command if it recognizes this response.

   PARSE  The human-readable text represents an error in parsing the
      [RFC-2822] header or [MIME-IMB] headers of a message in the
      mailbox.

   PERMANENTFLAGS  Followed by a parenthesized list of flags, indicates
      which of the known flags the client can change permanently.  Any
      flags that are in the FLAGS untagged response, but not the
      PERMANENTFLAGS list, can not be set permanently.  If the client
      attempts to STORE a flag that is not in the PERMANENTFLAGS list,
      the server will either ignore the change or store the state change
      for the remainder of the current session only.  The PERMANENTFLAGS
      list can also include the special flag \*, which indicates that it
      is possible to create new keywords by attempting to store those
      flags in the mailbox.

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   READ-ONLY  The mailbox is selected read-only, or its access while
      selected has changed from read-write to read-only.

   READ-WRITE  The mailbox is selected read-write, or its access while
      selected has changed from read-only to read-write.

   TRYCREATE  An APPEND or COPY attempt is failing because the target
      mailbox does not exist (as opposed to some other reason).  This is
      a hint to the client that the operation can succeed if the mailbox
      is first created by the CREATE command.

   UIDNEXT  Followed by a decimal number, indicates the next unique
      identifier value.  Refer to section 2.3.1.1 for more information.

   UIDVALIDITY  Followed by a decimal number, indicates the unique
      identifier validity value.  Refer to section 2.3.1.1 for more
      information.

   UNSEEN  Followed by a decimal number, indicates the number of the
      first message without the \Seen flag set.

   Additional response codes defined by particular client or server
   implementations SHOULD be prefixed with an "X" until they are added
   to a revision of this protocol.  Client implementations SHOULD ignore
   response codes that they do not recognize.

7.1.1.  OK Response

   Contents:   OPTIONAL response code
               human-readable text

   The OK response indicates an information message from the server.
   When tagged, it indicates successful completion of the associated
   command.  The human-readable text MAY be presented to the user as an
   information message.  The untagged form indicates an information-only
   message; the nature of the information MAY be indicated by a response
   code.

   The untagged form is also used as one of three possible greetings at
   connection startup.  It indicates that the connection is not yet
   authenticated and that a LOGIN command is needed.

      Example:    S: * OK IMAP4rev1 server ready
                  C: A001 LOGIN fred blurdybloop
                  S: * OK [ALERT] System shutdown in 10 minutes
                  S: A001 OK LOGIN Completed

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7.1.2.  NO Response

   Contents:   OPTIONAL response code
               human-readable text

   The NO response indicates an operational error message from the
   server.  When tagged, it indicates unsuccessful completion of the
   associated command.  The untagged form indicates a warning; the
   command can still complete successfully.  The human-readable text
   describes the condition.

    Example:    C: A222 COPY 1:2 owatagusiam
                S: * NO Disk is 98% full, please delete unnecessary data
                S: A222 OK COPY completed
                C: A223 COPY 3:200 blurdybloop
                S: * NO Disk is 98% full, please delete unnecessary data
                S: * NO Disk is 99% full, please delete unnecessary data
                S: A223 NO COPY failed: disk is full

7.1.3.  BAD Response

   Contents:   OPTIONAL response code
               human-readable text

   The BAD response indicates an error message from the server.  When
   tagged, it reports a protocol-level error in the client's command;
   the tag indicates the command that caused the error.  The untagged
   form indicates a protocol-level error for which the associated
   command can not be determined; it can also indicate an internal
   server failure.  The human-readable text describes the condition.

      Example:    C: ...very long command line...
                  S: * BAD Command line too long
                  C: ...empty line...
                  S: * BAD Empty command line
                  C: A443 EXPUNGE
                  S: * BAD Disk crash, attempting salvage to a new disk!
                  S: * OK Salvage successful, no data lost
                  S: A443 OK Expunge completed

7.1.4.  PREAUTH Response

   Contents:   OPTIONAL response code
               human-readable text

   The PREAUTH response is always untagged, and is one of three possible
   greetings at connection startup.  It indicates that the connection

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   has already been authenticated by external means; thus no LOGIN
   command is needed.

      Example:    S: * PREAUTH IMAP4rev1 server logged in as Smith

7.1.5.  BYE Response

   Contents:   OPTIONAL response code
               human-readable text

   The BYE response is always untagged, and indicates that the server is
   about to close the connection.  The human-readable text MAY be
   displayed to the user in a status report by the client.  The BYE
   response is sent under one of four conditions:

   1.  as part of a normal logout sequence.  The server will close the
       connection after sending the tagged OK response to the LOGOUT
       command.

   2.  as a panic shutdown announcement.  The server closes the
       connection immediately.

   3.  as an announcement of an inactivity autologout.  The server
       closes the connection immediately.

   4.  as one of three possible greetings at connection startup,
       indicating that the server is not willing to accept a connection
       from this client.  The server closes the connection immediately.

   The difference between a BYE that occurs as part of a normal LOGOUT
   sequence (the first case) and a BYE that occurs because of a failure
   (the other three cases) is that the connection closes immediately in
   the failure case.  In all cases the client SHOULD continue to read
   response data from the server until the connection is closed; this
   will ensure that any pending untagged or completion responses are
   read and processed.

      Example:    S: * BYE Autologout; idle for too long

7.2.  Server Responses - Server and Mailbox Status

   These responses are always untagged.  This is how server and mailbox
   status data are transmitted from the server to the client.  Many of
   these responses typically result from a command with the same name.

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7.2.1.  CAPABILITY Response

   Contents:   capability listing

   The CAPABILITY response occurs as a result of a CAPABILITY command.
   The capability listing contains a space-separated listing of
   capability names that the server supports.  The capability listing
   MUST include the atom "IMAP4rev1".

   In addition, client and server implementations MUST implement the
   STARTTLS, LOGINDISABLED, and AUTH=PLAIN (described in [IMAP-TLS])
   capabilities.  See the Security Considerations section for important
   information.

   A capability name which begins with "AUTH=" indicates that the server
   supports that particular authentication mechanism.

   The LOGINDISABLED capability indicates that the LOGIN command is
   disabled, and that the server will respond with a tagged NO response
   to any attempt to use the LOGIN command even if the user name and
   password are valid.  An IMAP client MUST NOT issue the LOGIN command
   if the server advertises the LOGINDISABLED capability.

   Other capability names indicate that the server supports an
   extension, revision, or amendment to the IMAP4rev1 protocol.  Server
   responses MUST conform to this document until the client issues a
   command that uses the associated capability.

   Capability names MUST either begin with "X" or be standard or
   standards-track IMAP4rev1 extensions, revisions, or amendments
   registered with IANA.  A server MUST NOT offer unregistered or non-
   standard capability names, unless such names are prefixed with an
   "X".

   Client implementations SHOULD NOT require any capability name other
   than "IMAP4rev1", and MUST ignore any unknown capability names.

   A server MAY send capabilities automatically, by using the CAPABILITY
   response code in the initial PREAUTH or OK responses, and by sending
   an updated CAPABILITY response code in the tagged OK response as part
   of a successful authentication.  It is unnecessary for a client to
   send a separate CAPABILITY command if it recognizes these automatic
   capabilities.

   Example:    S: * CAPABILITY IMAP4rev1 STARTTLS AUTH=GSSAPI XPIG-LATIN

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7.2.2.  LIST Response

   Contents:   name attributes
               hierarchy delimiter
               name

   The LIST response occurs as a result of a LIST command.  It returns a
   single name that matches the LIST specification.  There can be
   multiple LIST responses for a single LIST command.

   Four name attributes are defined:

   \Noinferiors  It is not possible for any child levels of hierarchy to
      exist under this name; no child levels exist now and none can be
      created in the future.

   \Noselect  It is not possible to use this name as a selectable
      mailbox.

   \Marked  The mailbox has been marked "interesting" by the server; the
      mailbox probably contains messages that have been added since the
      last time the mailbox was selected.

   \Unmarked  The mailbox does not contain any additional messages since
      the last time the mailbox was selected.

   If it is not feasible for the server to determine whether or not the
   mailbox is "interesting", or if the name is a \Noselect name, the
   server SHOULD NOT send either \Marked or \Unmarked.

   The hierarchy delimiter is a character used to delimit levels of
   hierarchy in a mailbox name.  A client can use it to create child
   mailboxes, and to search higher or lower levels of naming hierarchy.
   All children of a top-level hierarchy node MUST use the same
   separator character.  A NIL hierarchy delimiter means that no
   hierarchy exists; the name is a "flat" name.

   The name represents an unambiguous left-to-right hierarchy, and MUST
   be valid for use as a reference in LIST and LSUB commands.  Unless
   \Noselect is indicated, the name MUST also be valid as an argument
   for commands, such as SELECT, that accept mailbox names.

      Example:    S: * LIST (\Noselect) "/" ~/Mail/foo

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7.2.3.  LSUB Response

   Contents:   name attributes
               hierarchy delimiter
               name

   The LSUB response occurs as a result of an LSUB command.  It returns
   a single name that matches the LSUB specification.  There can be
   multiple LSUB responses for a single LSUB command.  The data is
   identical in format to the LIST response.

      Example:    S: * LSUB () "." #news.comp.mail.misc

7.2.4.  STATUS Response

   Contents:   name
               status parenthesized list

   The STATUS response occurs as a result of an STATUS command.  It
   returns the mailbox name that matches the STATUS specification and
   the requested mailbox status information.

      Example:    S: * STATUS blurdybloop (MESSAGES 231 UIDNEXT 44292)

7.2.5.  SEARCH Response

   Contents:   zero or more numbers

   The SEARCH response occurs as a result of a SEARCH or UID SEARCH
   command.  The number(s) refer to those messages that match the search
   criteria.  For SEARCH, these are message sequence numbers; for UID
   SEARCH, these are unique identifiers.  Each number is delimited by a
   space.

      Example:    S: * SEARCH 2 3 6

7.2.6.  FLAGS Response

   Contents:   flag parenthesized list

   The FLAGS response occurs as a result of a SELECT or EXAMINE command.
   The flag parenthesized list identifies the flags (at a minimum, the
   system-defined flags) that are applicable for this mailbox.  Flags
   other than the system flags can also exist, depending on server
   implementation.

   The update from the FLAGS response MUST be recorded by the client.

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      Example:    S: * FLAGS (\Answered \Flagged \Deleted \Seen \Draft)

7.3.  Server Responses - Mailbox Size

   These responses are always untagged.  This is how changes in the size
   of the mailbox are transmitted from the server to the client.
   Immediately following the "*" token is a number that represents a
   message count.

7.3.1.  EXISTS Response

   Contents:   none

   The EXISTS response reports the number of messages in the mailbox.
   This response occurs as a result of a SELECT or EXAMINE command, and
   if the size of the mailbox changes (e.g., new messages).

   The update from the EXISTS response MUST be recorded by the client.

      Example:    S: * 23 EXISTS

7.3.2.  RECENT Response

   Contents:   none

   The RECENT response reports the number of messages with the \Recent
   flag set.  This response occurs as a result of a SELECT or EXAMINE
   command, and if the size of the mailbox changes (e.g., new messages).

      Note: It is not guaranteed that the message sequence numbers of
      recent messages will be a contiguous range of the highest n
      messages in the mailbox (where n is the value reported by the
      RECENT response).  Examples of situations in which this is not the
      case are: multiple clients having the same mailbox open (the first
      session to be notified will see it as recent, others will probably
      see it as non-recent), and when the mailbox is re-ordered by a
      non-IMAP agent.

      The only reliable way to identify recent messages is to look at
      message flags to see which have the \Recent flag set, or to do a
      SEARCH RECENT.

   The update from the RECENT response MUST be recorded by the client.

      Example:    S: * 5 RECENT

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7.4.  Server Responses - Message Status

   These responses are always untagged.  This is how message data are
   transmitted from the server to the client, often as a result of a
   command with the same name.  Immediately following the "*" token is a
   number that represents a message sequence number.

7.4.1.  EXPUNGE Response

   Contents:   none

   The EXPUNGE response reports that the specified message sequence
   number has been permanently removed from the mailbox.  The message
   sequence number for each successive message in the mailbox is
   immediately decremented by 1, and this decrement is reflected in
   message sequence numbers in subsequent responses (including other
   untagged EXPUNGE responses).

   The EXPUNGE response also decrements the number of messages in the
   mailbox; it is not necessary to send an EXISTS response with the new
   value.

   As a result of the immediate decrement rule, message sequence numbers
   that appear in a set of successive EXPUNGE responses depend upon
   whether the messages are removed starting from lower numbers to
   higher numbers, or from higher numbers to lower numbers.  For
   example, if the last 5 messages in a 9-message mailbox are expunged,
   a "lower to higher" server will send five untagged EXPUNGE responses
   for message sequence number 5, whereas a "higher to lower server"
   will send successive untagged EXPUNGE responses for message sequence
   numbers 9, 8, 7, 6, and 5.

   An EXPUNGE response MUST NOT be sent when no command is in progress,
   nor while responding to a FETCH, STORE, or SEARCH command.  This rule
   is necessary to prevent a loss of synchronization of message sequence
   numbers between client and server.  A command is not "in progress"
   until the complete command has been received; in particular, a
   command is not "in progress" during the negotiation of command
   continuation.

      Note: UID FETCH, UID STORE, and UID SEARCH are different commands
      from FETCH, STORE, and SEARCH.  An EXPUNGE response MAY be sent
      during a UID command.

   The update from the EXPUNGE response MUST be recorded by the client.

      Example:    S: * 44 EXPUNGE

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7.4.2.  FETCH Response

   Contents:   message data

   The FETCH response returns data about a message to the client.  The
   data are pairs of data item names and their values in parentheses.
   This response occurs as the result of a FETCH or STORE command, as
   well as by unilateral server decision (e.g., flag updates).

   The current data items are:

   BODY  A form of BODYSTRUCTURE without extension data.

   BODY[<section>]<<origin octet>>

         A string expressing the body contents of the specified section.
         The string SHOULD be interpreted by the client according to the
         content transfer encoding, body type, and subtype.

         If the origin octet is specified, this string is a substring of
         the entire body contents, starting at that origin octet.  This
         means that BODY[]<0> MAY be truncated, but BODY[] is NEVER
         truncated.

            Note: The origin octet facility MUST NOT be used by a server
            in a FETCH response unless the client specifically requested
            it by means of a FETCH of a BODY[<section>]<<partial>> data
            item.

         8-bit textual data is permitted if a [CHARSET] identifier is
         part of the body parameter parenthesized list for this section.
         Note that headers (part specifiers HEADER or MIME, or the
         header portion of a MESSAGE/RFC822 part), MUST be 7-bit; 8-bit
         characters are not permitted in headers.  Note also that the
         [RFC-2822] delimiting blank line between the header and the
         body is not affected by header line subsetting; the blank line
         is always included as part of header data, except in the case
         of a message which has no body and no blank line.

         Non-textual data such as binary data MUST be transfer encoded
         into a textual form, such as BASE64, prior to being sent to the
         client.  To derive the original binary data, the client MUST
         decode the transfer encoded string.

   BODYSTRUCTURE

         A parenthesized list that describes the [MIME-IMB] body
         structure of a message.  This is computed by the server by

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         parsing the [MIME-IMB] header fields, defaulting various fields
         as necessary.

         For example, a simple text message of 48 lines and 2279 octets
         can have a body structure of: ("TEXT" "PLAIN" ("CHARSET" "US-
         ASCII") NIL NIL "7BIT" 2279 48)

         Multiple parts are indicated by parenthesis nesting.  Instead
         of a body type as the first element of the parenthesized list,
         there is a sequence of one or more nested body structures.  The
         second element of the parenthesized list is the multipart
         subtype (mixed, digest, parallel, alternative, etc.).

         For example, a two part message consisting of a text and a
         BASE64-encoded text attachment can have a body structure of:
         (("TEXT" "PLAIN" ("CHARSET" "US-ASCII") NIL NIL "7BIT" 1152
         23)("TEXT" "PLAIN" ("CHARSET" "US-ASCII" "NAME" "cc.diff")
         "<960723163407.20117h@cac.washington.edu>" "Compiler diff"
         "BASE64" 4554 73) "MIXED")

         Extension data follows the multipart subtype.  Extension data
         is never returned with the BODY fetch, but can be returned with
         a BODYSTRUCTURE fetch.  Extension data, if present, MUST be in
         the defined order.  The extension data of a multipart body part
         are in the following order:

         body parameter parenthesized list  A parenthesized list of
            attribute/value pairs [e.g., ("foo" "bar" "baz" "rag") where
            "bar" is the value of "foo", and "rag" is the value of
            "baz"] as defined in [MIME-IMB].

         body disposition  A parenthesized list, consisting of a
            disposition type string, followed by a parenthesized list of
            disposition attribute/value pairs as defined in
            [DISPOSITION].

         body language  A string or parenthesized list giving the body
            language value as defined in [LANGUAGE-TAGS].

         body location  A string list giving the body content URI as
            defined in [LOCATION].

         Any following extension data are not yet defined in this
         version of the protocol.  Such extension data can consist of
         zero or more NILs, strings, numbers, or potentially nested
         parenthesized lists of such data.  Client implementations that
         do a BODYSTRUCTURE fetch MUST be prepared to accept such
         extension data.  Server implementations MUST NOT send such

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         extension data until it has been defined by a revision of this
         protocol.

         The basic fields of a non-multipart body part are in the
         following order:

         body type  A string giving the content media type name as
            defined in [MIME-IMB].

         body subtype  A string giving the content subtype name as
            defined in [MIME-IMB].

         body parameter parenthesized list  A parenthesized list of
            attribute/value pairs [e.g., ("foo" "bar" "baz" "rag") where
            "bar" is the value of "foo" and "rag" is the value of "baz"]
            as defined in [MIME-IMB].

         body id  A string giving the content id as defined in
            [MIME-IMB].

         body description  A string giving the content description as
            defined in [MIME-IMB].

         body encoding  A string giving the content transfer encoding as
            defined in [MIME-IMB].

         body size  A number giving the size of the body in octets.
            Note that this size is the size in its transfer encoding and
            not the resulting size after any decoding.

         A body type of type MESSAGE and subtype RFC822 contains,
         immediately after the basic fields, the envelope structure,
         body structure, and size in text lines of the encapsulated
         message.

         A body type of type TEXT contains, immediately after the basic
         fields, the size of the body in text lines.  Note that this
         size is the size in its content transfer encoding and not the
         resulting size after any decoding.

         Extension data follows the basic fields and the type-specific
         fields listed above.  Extension data is never returned with the
         BODY fetch, but can be returned with a BODYSTRUCTURE fetch.
         Extension data, if present, MUST be in the defined order.

         The extension data of a non-multipart body part are in the
         following order:

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         body MD5  A string giving the body MD5 value as defined in
            [MD5].

         body disposition  A parenthesized list with the same content
            and function as the body disposition for a multipart body
            part.

         body language  A string or parenthesized list giving the body
            language value as defined in [LANGUAGE-TAGS].

         body location  A string list giving the body content URI as
            defined in [LOCATION].

         Any following extension data are not yet defined in this
         version of the protocol, and would be as described above under
         multipart extension data.

   ENVELOPE

         A parenthesized list that describes the envelope structure of a
         message.  This is computed by the server by parsing the
         [RFC-2822] header into the component parts, defaulting various
         fields as necessary.

         The fields of the envelope structure 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 parenthesized lists of address
         structures.

         An address structure is a parenthesized list that describes an
         electronic mail address.  The fields of an address structure
         are in the following order: personal name, [SMTP] at-domain-
         list (source route), mailbox name, and host name.

         [RFC-2822] group syntax is indicated by a special form of
         address structure in which the host name field is NIL.  If the
         mailbox name field is also NIL, this is an end of group marker
         (semi-colon in RFC 822 syntax).  If the mailbox name field is
         non-NIL, this is a start of group marker, and the mailbox name
         field holds the group name phrase.

         If the Date, Subject, In-Reply-To, and Message-ID header lines
         are absent in the [RFC-2822] header, the corresponding member
         of the envelope is NIL; if these header lines are present but
         empty the corresponding member of the envelope is the empty
         string.

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            Note: some servers may return a NIL envelope member in the
            "present but empty" case.  Clients SHOULD treat NIL and
            empty string as identical.

            Note: [RFC-2822] requires that all messages have a valid
            Date header.  Therefore, the date member in the envelope can
            not be NIL or the empty string.

            Note: [RFC-2822] requires that the In-Reply-To and Message-
            ID headers, if present, have non-empty content.  Therefore,
            the in-reply-to and message-id members in the envelope can
            not be the empty string.

         If the From, To, cc, and bcc header lines are absent in the
         [RFC-2822] header, or are present but empty, the corresponding
         member of the envelope is NIL.

         If the Sender or Reply-To lines are absent in the [RFC-2822]
         header, or are present but empty, the server sets the
         corresponding member of the envelope to be the same value as
         the from member (the client is not expected to know to do
         this).

            Note: [RFC-2822] requires that all messages have a valid
            From header.  Therefore, the from, sender, and reply-to
            members in the envelope can not be NIL.

   FLAGS  A parenthesized list of flags that are set for this message.

   INTERNALDATE  A string representing the internal date of the message.

   RFC822  Equivalent to BODY[].

   RFC822.HEADER  Equivalent to BODY[HEADER].  Note that this did not
      result in \Seen being set, because RFC822.HEADER response data
      occurs as a result of a FETCH of RFC822.HEADER.  BODY[HEADER]
      response data occurs as a result of a FETCH of BODY[HEADER] (which
      sets \Seen) or BODY.PEEK[HEADER] (which does not set \Seen).

   RFC822.SIZE  A number expressing the [RFC-2822] size of the message.

   RFC822.TEXT  Equivalent to BODY[TEXT].

   UID  A number expressing the unique identifier of the message.

      Example:    S: * 23 FETCH (FLAGS (\Seen) RFC822.SIZE 44827)

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7.5.  Server Responses - Command Continuation Request

   The command continuation request response is indicated by a "+" token
   instead of a tag.  This form of response indicates that the server is
   ready to accept the continuation of a command from the client.  The
   remainder of this response is a line of text.

   This response is used in the AUTHENTICATE command to transmit server
   data to the client, and request additional client data.  This
   response is also used if an argument to any command is a literal.

   The client is not permitted to send the octets of the literal unless
   the server indicates that it is expected.  This permits the server to
   process commands and reject errors on a line-by-line basis.  The
   remainder of the command, including the CRLF that terminates a
   command, follows the octets of the literal.  If there are any
   additional command arguments, the literal octets are followed by a
   space and those arguments.

      Example:    C: A001 LOGIN {11}
                  S: + Ready for additional command text
                  C: FRED FOOBAR {7}
                  S: + Ready for additional command text
                  C: fat man
                  S: A001 OK LOGIN completed
                  C: A044 BLURDYBLOOP {102856}
                  S: A044 BAD No such command as "BLURDYBLOOP"

8.  Sample IMAP4rev1 connection

   The following is a transcript of an IMAP4rev1 connection.  A long
   line in this sample is broken for editorial clarity.

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S:   * OK IMAP4rev1 Service Ready
C:   a001 login mrc secret
S:   a001 OK LOGIN completed
C:   a002 select inbox
S:   * 18 EXISTS
S:   * FLAGS (\Answered \Flagged \Deleted \Seen \Draft)
S:   * 2 RECENT
S:   * OK [UNSEEN 17] Message 17 is the first unseen message
S:   * OK [UIDVALIDITY 3857529045] UIDs valid
S:   a002 OK [READ-WRITE] SELECT completed
C:   a003 fetch 12 full
S:   * 12 FETCH (FLAGS (\Seen) INTERNALDATE "17-Jul-1996 02:44:25 -0700"
      RFC822.SIZE 4286 ENVELOPE ("Wed, 17 Jul 1996 02:23:25 -0700 (PDT)"
      "IMAP4rev1 WG mtg summary and minutes"
      (("Terry Gray" NIL "gray" "cac.washington.edu"))
      (("Terry Gray" NIL "gray" "cac.washington.edu"))
      (("Terry Gray" NIL "gray" "cac.washington.edu"))
      ((NIL NIL "imap" "cac.washington.edu"))
      ((NIL NIL "minutes" "CNRI.Reston.VA.US")
      ("John Klensin" NIL "KLENSIN" "MIT.EDU")) NIL NIL
      "<B27397-0100000@cac.washington.edu>")
       BODY ("TEXT" "PLAIN" ("CHARSET" "US-ASCII") NIL NIL "7BIT" 3028
       92))
S:    a003 OK FETCH completed
C:    a004 fetch 12 body[header]
S:    * 12 FETCH (BODY[HEADER] {342}
S:    Date: Wed, 17 Jul 1996 02:23:25 -0700 (PDT)
S:    From: Terry Gray <gray@cac.washington.edu>
S:    Subject: IMAP4rev1 WG mtg summary and minutes
S:    To: imap@cac.washington.edu
S:    cc: minutes@CNRI.Reston.VA.US, John Klensin <KLENSIN@MIT.EDU>
S:    Message-Id: <B27397-0100000@cac.washington.edu>
S:    MIME-Version: 1.0
S:    Content-Type: TEXT/PLAIN; CHARSET=US-ASCII
S:
S:    )
S:    a004 OK FETCH completed
C:    a005 store 12 +flags \deleted
S:    * 12 FETCH (FLAGS (\Seen \Deleted))
S:    a005 OK +FLAGS completed
C:    a006 logout
S:    * BYE IMAP4rev1 server terminating connection
S:    a006 OK LOGOUT completed

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9.  Formal Syntax

   The following syntax specification uses the Augmented Backus-Naur
   Form (ABNF) notation as specified in [ABNF].

   In the case of alternative or optional rules in which a later rule
   overlaps an earlier rule, the rule which is listed earlier MUST take
   priority.  For example, "\Seen" when parsed as a flag is the \Seen
   flag name and not a flag-extension, even though "\Seen" can be parsed
   as a flag-extension.  Some, but not all, instances of this rule are
   noted below.

      Note: [ABNF] rules MUST be followed strictly; in particular:

      (1) Except as noted otherwise, all alphabetic characters are case-
      insensitive.  The use of upper or lower case characters to define
      token strings is for editorial clarity only.  Implementations MUST
      accept these strings in a case-insensitive fashion.

      (2) In all cases, SP refers to exactly one space.  It is NOT
      permitted to substitute TAB, insert additional spaces, or
      otherwise treat SP as being equivalent to LWSP.

      (3) The ASCII NUL character, %x00, MUST NOT be used at any time.

address         = "(" addr-name SP addr-adl SP addr-mailbox SP
                  addr-host ")"

addr-adl        = nstring
                    ; Holds route from [RFC-2822] route-addr if
                    ; non-NIL

addr-host       = nstring
                    ; NIL indicates [RFC-2822] group syntax.
                    ; Otherwise, holds [RFC-2822] domain name

addr-mailbox    = nstring
                    ; NIL indicates end of [RFC-2822] group; if
                    ; non-NIL and addr-host is NIL, holds
                    ; [RFC-2822] group name.
                    ; Otherwise, holds [RFC-2822] local-part
                    ; after removing [RFC-2822] quoting

addr-name       = nstring
                    ; If non-NIL, holds phrase from [RFC-2822]
                    ; mailbox after removing [RFC-2822] quoting

append          = "APPEND" SP mailbox [SP flag-list] [SP date-time] SP

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                  literal

astring         = 1*ASTRING-CHAR / string

ASTRING-CHAR   = ATOM-CHAR / resp-specials

atom            = 1*ATOM-CHAR

ATOM-CHAR       = <any CHAR except atom-specials>

atom-specials   = "(" / ")" / "{" / SP / CTL / list-wildcards /
                  quoted-specials / resp-specials

authenticate    = "AUTHENTICATE" SP auth-type *(CRLF base64)

auth-type       = atom
                    ; Defined by [SASL]

base64          = *(4base64-char) [base64-terminal]

base64-char     = ALPHA / DIGIT / "+" / "/"
                    ; Case-sensitive

base64-terminal = (2base64-char "==") / (3base64-char "=")

body            = "(" (body-type-1part / body-type-mpart) ")"

body-extension  = nstring / number /
                   "(" body-extension *(SP body-extension) ")"
                    ; Future expansion.  Client implementations
                    ; MUST accept body-extension fields.  Server
                    ; implementations MUST NOT generate
                    ; body-extension fields except as defined by
                    ; future standard or standards-track
                    ; revisions of this specification.

body-ext-1part  = body-fld-md5 [SP body-fld-dsp [SP body-fld-lang
                  [SP body-fld-loc *(SP body-extension)]]]
                    ; MUST NOT be returned on non-extensible
                    ; "BODY" fetch

body-ext-mpart  = body-fld-param [SP body-fld-dsp [SP body-fld-lang
                  [SP body-fld-loc *(SP body-extension)]]]
                    ; MUST NOT be returned on non-extensible
                    ; "BODY" fetch

body-fields     = body-fld-param SP body-fld-id SP body-fld-desc SP
                  body-fld-enc SP body-fld-octets

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body-fld-desc   = nstring

body-fld-dsp    = "(" string SP body-fld-param ")" / nil

body-fld-enc    = (DQUOTE ("7BIT" / "8BIT" / "BINARY" / "BASE64"/
                  "QUOTED-PRINTABLE") DQUOTE) / string

body-fld-id     = nstring

body-fld-lang   = nstring / "(" string *(SP string) ")"

body-fld-loc    = nstring

body-fld-lines  = number

body-fld-md5    = nstring

body-fld-octets = number

body-fld-param  = "(" string SP string *(SP string SP string) ")" / nil

body-type-1part = (body-type-basic / body-type-msg / body-type-text)
                  [SP body-ext-1part]

body-type-basic = media-basic SP body-fields
                    ; MESSAGE subtype MUST NOT be "RFC822"

body-type-mpart = 1*body SP media-subtype
                  [SP body-ext-mpart]

body-type-msg   = media-message SP body-fields SP envelope
                  SP body SP body-fld-lines

body-type-text  = media-text SP body-fields SP body-fld-lines

capability      = ("AUTH=" auth-type) / atom
                    ; New capabilities MUST begin with "X" or be
                    ; registered with IANA as standard or
                    ; standards-track

capability-data = "CAPABILITY" *(SP capability) SP "IMAP4rev1"
                  *(SP capability)
                    ; Servers MUST implement the STARTTLS, AUTH=PLAIN,
                    ; and LOGINDISABLED capabilities
                    ; Servers which offer RFC 1730 compatibility MUST
                    ; list "IMAP4" as the first capability.

CHAR8           = %x01-ff

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                    ; any OCTET except NUL, %x00

command         = tag SP (command-any / command-auth / command-nonauth /
                  command-select) CRLF
                    ; Modal based on state

command-any     = "CAPABILITY" / "LOGOUT" / "NOOP" / x-command
                    ; Valid in all states

command-auth    = append / create / delete / examine / list / lsub /
                  rename / select / status / subscribe / unsubscribe
                    ; Valid only in Authenticated or Selected state

command-nonauth = login / authenticate / "STARTTLS"
                    ; Valid only when in Not Authenticated state

command-select  = "CHECK" / "CLOSE" / "EXPUNGE" / copy / fetch / store /
                  uid / search
                    ; Valid only when in Selected state

continue-req    = "+" SP (resp-text / base64) CRLF

copy            = "COPY" SP sequence-set SP mailbox

create          = "CREATE" SP mailbox
                    ; Use of INBOX gives a NO error

date            = date-text / DQUOTE date-text DQUOTE

date-day        = 1*2DIGIT
                    ; Day of month

date-day-fixed  = (SP DIGIT) / 2DIGIT
                    ; Fixed-format version of date-day

date-month      = "Jan" / "Feb" / "Mar" / "Apr" / "May" / "Jun" /
                  "Jul" / "Aug" / "Sep" / "Oct" / "Nov" / "Dec"

date-text       = date-day "-" date-month "-" date-year

date-year       = 4DIGIT

date-time       = DQUOTE date-day-fixed "-" date-month "-" date-year
                  SP time SP zone DQUOTE

delete          = "DELETE" SP mailbox
                    ; Use of INBOX gives a NO error

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digit-nz        = %x31-39
                    ; 1-9

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

env-cc          = "(" 1*address ")" / nil

env-date        = nstring

env-from        = "(" 1*address ")" / nil

env-in-reply-to = nstring

env-message-id  = nstring

env-reply-to    = "(" 1*address ")" / nil

env-sender      = "(" 1*address ")" / nil

env-subject     = nstring

env-to          = "(" 1*address ")" / nil

examine         = "EXAMINE" SP mailbox

fetch           = "FETCH" SP sequence-set SP ("ALL" / "FULL" / "FAST" /
                  fetch-att / "(" fetch-att *(SP fetch-att) ")")

fetch-att       = "ENVELOPE" / "FLAGS" / "INTERNALDATE" /
                  "RFC822" [".HEADER" / ".SIZE" / ".TEXT"] /
                  "BODY" ["STRUCTURE"] / "UID" /
                  "BODY" section ["<" number "." nz-number ">"] /
                  "BODY.PEEK" section ["<" number "." nz-number ">"]

flag            = "\Answered" / "\Flagged" / "\Deleted" /
                  "\Seen" / "\Draft" / flag-keyword / flag-extension
                    ; Does not include "\Recent"

flag-extension  = "\" atom
                    ; Future expansion.  Client implementations
                    ; MUST accept flag-extension flags.  Server
                    ; implementations MUST NOT generate
                    ; flag-extension flags except as defined by
                    ; future standard or standards-track

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                    ; revisions of this specification.

flag-fetch      = flag / "\Recent"

flag-keyword    = atom

flag-list       = "(" [flag *(SP flag)] ")"

flag-perm       = flag / "\*"

greeting        = "*" SP (resp-cond-auth / resp-cond-bye) CRLF

header-fld-name = astring

header-list     = "(" header-fld-name *(SP header-fld-name) ")"

list            = "LIST" SP mailbox SP list-mailbox

list-mailbox    = 1*list-char / string

list-char       = ATOM-CHAR / list-wildcards / resp-specials

list-wildcards  = "%" / "*"

literal         = "{" number "}" CRLF *CHAR8
                    ; Number represents the number of CHAR8s

login           = "LOGIN" SP userid SP password

lsub            = "LSUB" SP mailbox SP list-mailbox

mailbox         = "INBOX" / astring
                    ; INBOX is case-insensitive.  All case variants of
                    ; INBOX (e.g., "iNbOx") MUST be interpreted as INBOX
                    ; not as an astring.  An astring which consists of
                    ; the case-insensitive sequence "I" "N" "B" "O" "X"
                    ; is considered to be INBOX and not an astring.
                    ;  Refer to section 5.1 for further
                    ; semantic details of mailbox names.

mailbox-data    =  "FLAGS" SP flag-list / "LIST" SP mailbox-list /
                   "LSUB" SP mailbox-list / "SEARCH" *(SP nz-number) /
                   "STATUS" SP mailbox SP "(" [status-att-list] ")" /
                   number SP "EXISTS" / number SP "RECENT"

mailbox-list    = "(" [mbx-list-flags] ")" SP
                   (DQUOTE QUOTED-CHAR DQUOTE / nil) SP mailbox

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mbx-list-flags  = *(mbx-list-oflag SP) mbx-list-sflag
                  *(SP mbx-list-oflag) /
                  mbx-list-oflag *(SP mbx-list-oflag)

mbx-list-oflag  = "\Noinferiors" / flag-extension
                    ; Other flags; multiple possible per LIST response

mbx-list-sflag  = "\Noselect" / "\Marked" / "\Unmarked"
                    ; Selectability flags; only one per LIST response

media-basic     = ((DQUOTE ("APPLICATION" / "AUDIO" / "IMAGE" /
                  "MESSAGE" / "VIDEO") DQUOTE) / string) SP
                  media-subtype
                    ; Defined in [MIME-IMT]

media-message   = DQUOTE "MESSAGE" DQUOTE SP DQUOTE "RFC822" DQUOTE
                    ; Defined in [MIME-IMT]

media-subtype   = string
                    ; Defined in [MIME-IMT]

media-text      = DQUOTE "TEXT" DQUOTE SP media-subtype
                    ; Defined in [MIME-IMT]

message-data    = nz-number SP ("EXPUNGE" / ("FETCH" SP msg-att))

msg-att         = "(" (msg-att-dynamic / msg-att-static)
                   *(SP (msg-att-dynamic / msg-att-static)) ")"

msg-att-dynamic = "FLAGS" SP "(" [flag-fetch *(SP flag-fetch)] ")"
                    ; MAY change for a message

msg-att-static  = "ENVELOPE" SP envelope / "INTERNALDATE" SP date-time /
                  "RFC822" [".HEADER" / ".TEXT"] SP nstring /
                  "RFC822.SIZE" SP number /
                  "BODY" ["STRUCTURE"] SP body /
                  "BODY" section ["<" number ">"] SP nstring /
                  "UID" SP uniqueid
                    ; MUST NOT change for a message

nil             = "NIL"

nstring         = string / nil

number          = 1*DIGIT
                    ; Unsigned 32-bit integer
                    ; (0 <= n < 4,294,967,296)

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nz-number       = digit-nz *DIGIT
                    ; Non-zero unsigned 32-bit integer
                    ; (0 < n < 4,294,967,296)

password        = astring

quoted          = DQUOTE *QUOTED-CHAR DQUOTE

QUOTED-CHAR     = <any TEXT-CHAR except quoted-specials> /
                  "\" quoted-specials

quoted-specials = DQUOTE / "\"

rename          = "RENAME" SP mailbox SP mailbox
                    ; Use of INBOX as a destination gives a NO error

response        = *(continue-req / response-data) response-done

response-data   = "*" SP (resp-cond-state / resp-cond-bye /
                  mailbox-data / message-data / capability-data) CRLF

response-done   = response-tagged / response-fatal

response-fatal  = "*" SP resp-cond-bye CRLF
                    ; Server closes connection immediately

response-tagged = tag SP resp-cond-state CRLF

resp-cond-auth  = ("OK" / "PREAUTH") SP resp-text
                    ; Authentication condition

resp-cond-bye   = "BYE" SP resp-text

resp-cond-state = ("OK" / "NO" / "BAD") SP resp-text
                    ; Status condition

resp-specials   = "]"

resp-text       = ["[" resp-text-code "]" SP] text

resp-text-code  = "ALERT" /
                  "BADCHARSET" [SP "(" astring *(SP astring) ")" ] /
                  capability-data / "PARSE" /
                  "PERMANENTFLAGS" SP "("
                  [flag-perm *(SP flag-perm)] ")" /
                  "READ-ONLY" / "READ-WRITE" / "TRYCREATE" /
                  "UIDNEXT" SP nz-number / "UIDVALIDITY" SP nz-number /
                  "UNSEEN" SP nz-number /

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                  atom [SP 1*<any TEXT-CHAR except "]">]

search          = "SEARCH" [SP "CHARSET" SP astring] 1*(SP search-key)
                    ; CHARSET argument to MUST be registered with IANA

search-key      = "ALL" / "ANSWERED" / "BCC" SP astring /
                  "BEFORE" SP date / "BODY" SP astring /
                  "CC" SP astring / "DELETED" / "FLAGGED" /
                  "FROM" SP astring / "KEYWORD" SP flag-keyword /
                  "NEW" / "OLD" / "ON" SP date / "RECENT" / "SEEN" /
                  "SINCE" SP date / "SUBJECT" SP astring /
                  "TEXT" SP astring / "TO" SP astring /
                  "UNANSWERED" / "UNDELETED" / "UNFLAGGED" /
                  "UNKEYWORD" SP flag-keyword / "UNSEEN" /
                    ; Above this line were in [IMAP2]
                  "DRAFT" / "HEADER" SP header-fld-name SP astring /
                  "LARGER" SP number / "NOT" SP search-key /
                  "OR" SP search-key SP search-key /
                  "SENTBEFORE" SP date / "SENTON" SP date /
                  "SENTSINCE" SP date / "SMALLER" SP number /
                  "UID" SP sequence-set / "UNDRAFT" / sequence-set /
                  "(" search-key *(SP search-key) ")"

section         = "[" [section-spec] "]"

section-msgtext = "HEADER" / "HEADER.FIELDS" [".NOT"] SP header-list /
                  "TEXT"
                    ; top-level or MESSAGE/RFC822 part

section-part    = nz-number *("." nz-number)
                    ; body part nesting

section-spec    = section-msgtext / (section-part ["." section-text])

section-text    = section-msgtext / "MIME"
                    ; text other than actual body part (headers, etc.)

select          = "SELECT" SP mailbox

seq-number      = nz-number / "*"
                    ; message sequence number (COPY, FETCH, STORE
                    ; commands) or unique identifier (UID COPY,
                    ; UID FETCH, UID STORE commands).
                    ; * represents the largest number in use.  In
                    ; the case of message sequence numbers, it is
                    ; the number of messages in a non-empty mailbox.
                    ; In the case of unique identifiers, it is the
                    ; unique identifier of the last message in the

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                    ; mailbox or, if the mailbox is empty, the
                    ; mailbox's current UIDNEXT value.
                    ; The server should respond with a tagged BAD
                    ; response to a command that uses a message
                    ; sequence number greater than the number of
                    ; messages in the selected mailbox.  This
                    ; includes "*" if the selected mailbox is empty.

seq-range       = seq-number ":" seq-number
                    ; two seq-number values and all values between
                    ; these two regardless of order.
                    ; Example: 2:4 and 4:2 are equivalent and indicate
                    ; values 2, 3, and 4.
                    ; Example: a unique identifier sequence range of
                    ; 3291:* includes the UID of the last message in
                    ; the mailbox, even if that value is less than 3291.

sequence-set    = (seq-number / seq-range) *("," sequence-set)
                    ; set of seq-number values, regardless of order.
                    ; Servers MAY coalesce overlaps and/or execute the
                    ; sequence in any order.
                    ; Example: a message sequence number set of
                    ; 2,4:7,9,12:* for a mailbox with 15 messages is
                    ; equivalent to 2,4,5,6,7,9,12,13,14,15
                    ; Example: a message sequence number set of *:4,5:7
                    ; for a mailbox with 10 messages is equivalent to
                    ; 10,9,8,7,6,5,4,5,6,7 and MAY be reordered and
                    ; overlap coalesced to be 4,5,6,7,8,9,10.

status          = "STATUS" SP mailbox SP
                  "(" status-att *(SP status-att) ")"

status-att      = "MESSAGES" / "RECENT" / "UIDNEXT" / "UIDVALIDITY" /
                  "UNSEEN"

status-att-list =  status-att SP number *(SP status-att SP number)

store           = "STORE" SP sequence-set SP store-att-flags

store-att-flags = (["+" / "-"] "FLAGS" [".SILENT"]) SP
                  (flag-list / (flag *(SP flag)))

string          = quoted / literal

subscribe       = "SUBSCRIBE" SP mailbox

tag             = 1*<any ASTRING-CHAR except "+">

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text            = 1*TEXT-CHAR

TEXT-CHAR       = <any CHAR except CR and LF>

time            = 2DIGIT ":" 2DIGIT ":" 2DIGIT
                    ; Hours minutes seconds

uid             = "UID" SP (copy / fetch / search / store)
                    ; Unique identifiers used instead of message
                    ; sequence numbers

uniqueid        = nz-number
                    ; Strictly ascending

unsubscribe     = "UNSUBSCRIBE" SP mailbox

userid          = astring

x-command       = "X" atom <experimental command arguments>

zone            = ("+" / "-") 4DIGIT
                    ; Signed four-digit value of hhmm representing
                    ; hours and minutes east of Greenwich (that is,
                    ; the amount that the given time differs from
                    ; Universal Time).  Subtracting the timezone
                    ; from the given time will give the UT form.
                    ; The Universal Time zone is "+0000".

10.  Author's Note

   This document is a revision or rewrite of earlier documents, and
   supercedes the protocol specification in those documents: RFC 2060,
   RFC 1730, unpublished IMAP2bis.TXT document, RFC 1176, and RFC 1064.

11.  Security Considerations

   IMAP4rev1 protocol transactions, including electronic mail data, are
   sent in the clear over the network unless protection from snooping is
   negotiated.  This can be accomplished either by the use of STARTTLS,
   negotiated privacy protection in the AUTHENTICATE command, or some
   other protection mechanism.

11.1.  STARTTLS Security Considerations

   The specification of the STARTTLS command and LOGINDISABLED
   capability in this document replaces that in [IMAP-TLS].  [IMAP-TLS]
   remains normative for the PLAIN [SASL] authenticator.

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   IMAP client and server implementations MUST implement the
   TLS_RSA_WITH_RC4_128_MD5 [TLS] cipher suite, and SHOULD implement the
   TLS_DHE_DSS_WITH_3DES_EDE_CBC_SHA [TLS] cipher suite.  This is
   important as it assures that any two compliant implementations can be
   configured to interoperate.  All other cipher suites are OPTIONAL.
   Note that this is a change from section 2.1 of [IMAP-TLS].

   During the [TLS] negotiation, the client MUST check its understanding
   of the server hostname against the server's identity as presented in
   the server Certificate message, in order to prevent man-in-the-middle
   attacks.  If the match fails, the client SHOULD either ask for
   explicit user confirmation, or terminate the connection and indicate
   that the server's identity is suspect.  Matching is performed
   according to these rules:

      The client MUST use the server hostname it used to open the
      connection as the value to compare against the server name as
      expressed in the server certificate.  The client MUST NOT use any
      form of the server hostname derived from an insecure remote source
      (e.g., insecure DNS lookup).  CNAME canonicalization is not done.

      If a subjectAltName extension of type dNSName is present in the
      certificate, it SHOULD be used as the source of the server's
      identity.

      Matching is case-insensitive.

      A "*" wildcard character MAY be used as the left-most name
      component in the certificate.  For example, *.example.com would
      match a.example.com, foo.example.com, etc. but would not match
      example.com.

      If the certificate contains multiple names (e.g., more than one
      dNSName field), then a match with any one of the fields is
      considered acceptable.

   Both the client and server MUST check the result of the STARTTLS
   command and subsequent [TLS] negotiation to see whether acceptable
   authentication or privacy was achieved.

11.2.  Other Security Considerations

   A server error message for an AUTHENTICATE command which fails due to
   invalid credentials SHOULD NOT detail why the credentials are
   invalid.

   Use of the LOGIN command sends passwords in the clear.  This can be
   avoided by using the AUTHENTICATE command with a [SASL] mechanism

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   that does not use plaintext passwords, by first negotiating
   encryption via STARTTLS or some other protection mechanism.

   A server implementation MUST implement a configuration that, at the
   time of authentication, requires:
   (1) The STARTTLS command has been negotiated.
   OR
   (2) Some other mechanism that protects the session from password
   snooping has been provided.
   OR
   (3) The following measures are in place:
   (a) The LOGINDISABLED capability is advertised, and [SASL] mechanisms
   (such as PLAIN) using plaintext passwords are NOT advertised in the
   CAPABILITY list.
   AND
   (b) The LOGIN command returns an error even if the password is
   correct.
   AND
   (c) The AUTHENTICATE command returns an error with all [SASL]
   mechanisms that use plaintext passwords, even if the password is
   correct.

   A server error message for a failing LOGIN command SHOULD NOT specify
   that the user name, as opposed to the password, is invalid.

   A server SHOULD have mechanisms in place to limit or delay failed
   AUTHENTICATE/LOGIN attempts.

   Additional security considerations are discussed in the section
   discussing the AUTHENTICATE and LOGIN commands.

12.  IANA Considerations

   IMAP4 capabilities are registered by publishing a standards track or
   IESG approved experimental RFC.  The registry is currently located
   at: http://www.iana.org/assignments/imap4-capabilities

   As this specification revises the STARTTLS and LOGINDISABLED
   extensions previously defined in [IMAP-TLS], IANA is requested to
   update the registry accordingly.

13.  References

13.1.  Normative References

   [ABNF]     Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
              Specifications: ABNF", RFC 2234, November 1997,
              <http://www.rfc-editor.org/info/rfc2234>.

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   [ANONYMOUS]
              Newman, C., "Anonymous SASL Mechanism", RFC 2245, November
              1997, <http://www.rfc-editor.org/info/rfc2245>.

   [CHARSET]  Freed, N. and J. Postel, "IANA Charset Registration
              Procedures", BCP 19, RFC 2978, October 2000,
              <http://www.rfc-editor.org/info/rfc2978>.

   [DIGEST-MD5]
              Leach, P. and C. Newman, "Using Digest Authentication as a
              SASL Mechanism", RFC 2831, May 2000,
              <http://www.rfc-editor.org/info/rfc2831>.

   [DISPOSITION]
              Troost, R., Dorner, S., and K. Moore, Ed., "Communicating
              Presentation Information in Internet Messages: The
              Content-Disposition Header Field", RFC 2183, August 1997,
              <http://www.rfc-editor.org/info/rfc2183>.

   [IMAP-TLS]
              Newman, C., "Using TLS with IMAP, POP3 and ACAP", RFC
              2595, June 1999, <http://www.rfc-editor.org/info/rfc2595>.

   [KEYWORDS]
              Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997,
              <http://www.rfc-editor.org/info/rfc2119>.

   [LANGUAGE-TAGS]
              Alvestrand, H., "Tags for the Identification of
              Languages", RFC 3066, January 2001,
              <http://www.rfc-editor.org/info/rfc3066>.

   [LOCATION]
              Palme, J., Hopmann, A., and N. Shelness, "MIME
              Encapsulation of Aggregate Documents, such as HTML
              (MHTML)", RFC 2557, March 1999,
              <http://www.rfc-editor.org/info/rfc2557>.

   [MD5]      Myers, J. and M. Rose, "The Content-MD5 Header Field", RFC
              1864, October 1995,
              <http://www.rfc-editor.org/info/rfc1864>.

   [MIME-HDRS]
              Moore, K., "MIME (Multipurpose Internet Mail Extensions)
              Part Three: Message Header Extensions for Non-ASCII Text",
              RFC 2047, November 1996,
              <http://www.rfc-editor.org/info/rfc2047>.

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   [MIME-IMB]
              Freed, N. and N. Borenstein, "Multipurpose Internet Mail
              Extensions (MIME) Part One: Format of Internet Message
              Bodies", RFC 2045, November 1996,
              <http://www.rfc-editor.org/info/rfc2045>.

   [MIME-IMT]
              Freed, N. and N. Borenstein, "Multipurpose Internet Mail
              Extensions (MIME) Part Two: Media Types", RFC 2046,
              November 1996, <http://www.rfc-editor.org/info/rfc2046>.

   [RFC-2822]
              Resnick, P., Ed., "Internet Message Format", RFC 2822,
              April 2001, <http://www.rfc-editor.org/info/rfc2822>.

   [SASL]     Myers, J., "Simple Authentication and Security Layer
              (SASL)", RFC 2222, October 1997,
              <http://www.rfc-editor.org/info/rfc2222>.

   [TLS]      Dierks, T. and C. Allen, "The TLS Protocol Version 1.0",
              RFC 2246, January 1999,
              <http://www.rfc-editor.org/info/rfc2246>.

   [UTF-7]    Goldsmith, D. and M. Davis, "UTF-7 A Mail-Safe
              Transformation Format of Unicode", RFC 2152, May 1997,
              <http://www.rfc-editor.org/info/rfc2152>.

   [IMAP-IMPLEMENTATION]
              Leiba, B., "IMAP4 Implementation Recommendations", RFC
              2683, September 1999,
              <http://www.rfc-editor.org/info/rfc2683>.

   [IMAP-MULTIACCESS]
              Gahrns, M., "IMAP4 Multi-Accessed Mailbox Practice", RFC
              2180, July 1997, <http://www.rfc-editor.org/info/rfc2180>.

13.2.  Informative References (related protocols)

   [IMAP-DISC]
              Melnikov, A., Ed., "Synchronization Operations for
              Disconnected IMAP4 Clients", RFC 4549, June 2006,
              <http://www.rfc-editor.org/info/rfc4549>.

   [IMAP-MODEL]
              Crispin, M., "Distributed Electronic Mail Models in
              IMAP4", RFC 1733, December 1994,
              <http://www.rfc-editor.org/info/rfc1733>.

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   [ACAP]     Newman, C. and J. G. Myers, "ACAP -- Application
              Configuration Access Protocol", RFC 2244, November 1997,
              <http://www.rfc-editor.org/info/rfc2244>.

   [SMTP]     Klensin, J., Ed., "Simple Mail Transfer Protocol", RFC
              2821, April 2001,
              <http://www.rfc-editor.org/info/rfc2821>.

13.3.  Informative References (historical versions of IMAP)

   [IMAP-COMPAT]
              Crispin, M., "IMAP4 Compatibility with IMAP2bis", RFC
              2061, December 1996,
              <http://www.rfc-editor.org/info/rfc2061>.

   [IMAP-HISTORICAL]
              Crispin, M., "IMAP4 Compatibility with IMAP2 and
              IMAP2bis", RFC 1732, December 1994,
              <http://www.rfc-editor.org/info/rfc1732>.

   [IMAP-OBSOLETE]
              Crispin, M., "Internet Message Access Protocol - Obsolete
              Syntax", RFC 2062, December 1996,
              <http://www.rfc-editor.org/info/rfc2062>.

   [IMAP2]    Crispin, M., "Interactive Mail Access Protocol: Version
              2", RFC 1176, August 1990,
              <http://www.rfc-editor.org/info/rfc1176>.

   [RFC-822]  Crocker, D., "STANDARD FOR THE FORMAT OF ARPA INTERNET
              TEXT MESSAGES", STD 11, RFC 822, August 1982,
              <http://www.rfc-editor.org/info/rfc822>.

   [RFC-821]  Postel, J., "Simple Mail Transfer Protocol", STD 10, RFC
              821, August 1982, <http://www.rfc-editor.org/info/rfc821>.

Appendix A.  Changes from RFC 3501

   1.  Only formatting changes or changes that were a result of
       conversion to XML.

Appendix B.  Acknowledgement

   Earlier versions of this document were edited by Mark Crispin.
   Sadly, he is no longer available to help with this work.  Editor of
   this revisions is hoping that Mark would have approved.

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Appendix C.  Key Word Index

   [[CREF1: Note for RFC-Editor: can you generate the list of keywords
   and reference sections a la RFC 3501?]]

Author's Address

   Alexey Melnikov (editor)
   Isode Ltd
   14 Castle Mews
   Hampton, Middlesex  TW12 2NP
   UK

   Email: Alexey.Melnikov@isode.com

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