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6.      Client Commands

   IMAP4 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.

   Some commands cause specific server data to be returned; these are
   identified by "Data:" 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 data 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.


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

   Data:       mandatory 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 "IMAP4" as the first listed capability
      before the (tagged) OK response.  This listing of capabilities is
      not dependent upon connection state or user.  It is therefore not
      necessary to issue a CAPABILITY command more than once in a
      session.



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      Capability names other than "IMAP4" refer to extensions,
      revisions, or amendments to this specification.  See the
      documentation of the CAPABILITY response for additional
      information.  No capabilities are enabled without explicit client
      action to invoke the capability.  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 IMAP4
               S: abcd OK CAPABILITY completed


6.1.2.  NOOP Command

   Arguments:  none

   Data:       no specific data 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.  The NOOP
      command can also be used to reset any inactivity autologout timer
      on the server.

   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












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6.1.3.  LOGOUT Command

   Arguments:  none

   Data:       mandatory 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 session.  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 IMAP4 Server logging out
               S: A023 OK LOGOUT completed
               (Server and client then close the connection)



6.2.    Client Commands - Non-Authenticated State

   In non-authenticated state, the AUTHENTICATE or LOGIN command
   establishes authentication and enter authenticated state.  The
   AUTHENTICATE command provides a general mechanism for a variety of
   authentication techniques, whereas the LOGIN command uses the
   traditional user name and plaintext password pair.

   Server implementations may allow non-authenticated access to certain
   mailboxes.  The convention is to use a LOGIN command with the userid
   "anonymous".  A password is required.  It is implementation-dependent
   what requirements, if any, are placed on the password and what access
   restrictions are placed on anonymous users.

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

   In addition to the universal commands (CAPABILITY, NOOP, and LOGOUT),
   the following commands are valid in non-authenticated state:
   AUTHENTICATE and LOGIN.











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6.2.1.  AUTHENTICATE Command

   Arguments:  authentication mechanism name

   Data:       continuation data may 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 an authentication mechanism,
      such as described in [IMAP-AUTH], to the server.  If the server
      supports the requested authentication mechanism, it performs an
      authentication protocol exchange to authenticate and identify the
      user.  Optionally, it also negotiates a protection mechanism 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 authentication protocol exchange consists of a series of
      server challenges and client answers 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 answer consists of a line
      consisting of a BASE64 encoded string.  If the client wishes to
      cancel an authentication exchange, it should issue a line with a
      single "*".  If the server receives such an answer, it must reject
      the AUTHENTICATE command by sending a tagged BAD response.

      A protection mechanism provides integrity and privacy protection
      to the protocol session.  If a protection mechanism is negotiated,
      it is applied to all subsequent data sent over the connection.
      The protection mechanism 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.  Once the
      protection mechanism is in effect, the stream of command and
      response octets is processed into buffers of ciphertext.  Each
      buffer is transferred over the connection as a stream of octets
      prepended with a four octet field in network byte order that
      represents the length of the following data.  The maximum
      ciphertext buffer length is defined by the protection mechanism.

      The server is not required to support any particular
      authentication mechanism, nor are authentication mechanisms
      required to support any protection mechanisms.  If an AUTHENTICATE
      command fails with a NO response, the client may try another



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      authentication mechanism by issuing another AUTHENTICATE command,
      or may attempt to authenticate by using the LOGIN command.  In
      other words, the client may request authentication types in
      decreasing order of preference, with the LOGIN command as a last
      resort.

   Example:    S: * OK KerberosV4 IMAP4 Server
               C: A001 AUTHENTICATE KERBEROS_V4
               S: + AmFYig==
               C: BAcAQU5EUkVXLkNNVS5FRFUAOCAsho84kLN3/IJmrMG+25a4DT
                  +nZImJjnTNHJUtxAA+o0KPKfHEcAFs9a3CL5Oebe/ydHJUwYFd
                  WwuQ1MWiy6IesKvjL5rL9WjXUb9MwT9bpObYLGOKi1Qh
               S: + or//EoAADZI=
               C: DiAF5A4gA+oOIALuBkAAmw==
               S: A001 OK Kerberos V4 authentication successful

        Note: the line breaks in the first client answer are for
        editorial clarity and are not in real authenticators.


6.2.2.  LOGIN Command

   Arguments:  user name
               password

   Data:       no specific data 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 user to the server and carries
      the plaintext password authenticating this user.

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



6.3.    Client Commands - Authenticated State

   In 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 selected state.







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

6.3.1.  SELECT Command

   Arguments:  mailbox name

   Data:       mandatory untagged responses: FLAGS, EXISTS, RECENT
               optional OK untagged responses: UNSEEN, PERMANENTFLAGS

   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

      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:

         FLAGS       Defined flags in the mailbox

         <n> EXISTS  The number of messages in the mailbox

         <n> RECENT  The number of messages added to the  mailbox  since
                     the previous time this mailbox was read

         OK [UIDVALIDITY <n>]
                     The unique  identifier  validity  value.   See  the
                     description of the UID command for more detail.

      to define the initial state of the mailbox at the client.  If it
      is not possible to determine the messages that were added since
      the previous time a mailbox was read, then all messages SHOULD be
      considered recent.

      The server SHOULD also send an UNSEEN response code in an OK
      untagged response, indicating the message sequence number of the
      first unseen message in the mailbox.

      If the client can not change the permanent state of one or more of
      the flags listed in the FLAGS untagged response, the server SHOULD
      send a PERMANENTFLAGS response code in an OK untagged response,
      listing the flags that the client may change permanently.

      Only one mailbox may be selected at a time in a session;
      simultaneous access to multiple mailboxes requires multiple



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      sessions.  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 user 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 user is not permitted to modify the mailbox but is
      permitted read access, the mailbox is selected as read-only, and
      the server 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 user's .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: * FLAGS (\Answered \Flagged \Deleted \Seen \Draft)
               S: * OK [PERMANENTFLAGS (\Deleted \Seen \*)] Limited
               S: A142 OK [READ-WRITE] SELECT completed


6.3.2.  EXAMINE Command