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RFC 7162

 
 
 

IMAP Extensions: Quick Flag Changes Resynchronization (CONDSTORE) and Quick Mailbox Resynchronization (QRESYNC)

Part 2 of 2, p. 24 to 52
Prev RFC Part

 


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3.2.  QRESYNC Extension

   All protocol changes and requirements specified for the CONDSTORE
   extension are also a part of the QRESYNC extension.

   The QRESYNC extension puts additional requirements on a server
   implementing the CONDSTORE extension.  Each mailbox that supports
   persistent storage of mod-sequences, i.e., for which the server would
   send a HIGHESTMODSEQ untagged OK response code on a successful
   SELECT/EXAMINE, MUST increment the per-mailbox mod-sequence when one
   or more messages are expunged due to EXPUNGE, UID EXPUNGE, CLOSE, or
   MOVE [RFC6851]; the server MUST associate the incremented mod-
   sequence with the UIDs of the expunged messages.  Additionally, if
   the server also supports the IMAP METADATA extension [RFC5464], it
   MUST increment the per-mailbox mod-sequence when SETMETADATA
   successfully changes an annotation on the corresponding mailbox.

   A server implementing QRESYNC MUST send untagged events to a client
   in a way that the client doesn't lose any changes in case of
   connectivity loss.  In particular, this means that if the server

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   sends MODSEQ FETCH data items while EXPUNGE (or VANISHED) replies
   with lower mod-sequences being delayed, the server MUST send the
   HIGHESTMODSEQ response code with a lower value than the EXPUNGE's
   mod-sequence.  See the example in Section 6.

3.2.1.  Impact on CONDSTORE-only Clients

   A client that supports CONDSTORE but not QRESYNC might resynchronize
   a mailbox and discover that its HIGHESTMODSEQ has increased from the
   value cached by the client.  If the increase is only due to messages
   having been expunged since the client last synchronized, the client
   is likely to send a FETCH ... CHANGEDSINCE command that returns no
   data.  Thus, a client that supports CONDSTORE but not QRESYNC might
   incur a penalty of an unneeded round trip when resynchronizing some
   mailboxes (those that have had messages expunged but no flag changes
   since the last synchronization).

   This extra round trip is only incurred by clients that support
   CONDSTORE but not QRESYNC and only when a mailbox has had messages
   expunged but no flag changes to non-expunged messages.  Since
   CONDSTORE is a relatively new extension, it is strongly encouraged
   that clients that support it also support QRESYNC.

3.2.2.  Advertising Support for QRESYNC

   The quick resync IMAP extension is present if an IMAP4 server returns
   "QRESYNC" as one of the supported capabilities to the CAPABILITY
   command.

   For compatibility with clients that only support the CONDSTORE IMAP
   extension, servers SHOULD also advertise "CONDSTORE" in the
   CAPABILITY response.

3.2.3.  Use of ENABLE

   Servers supporting QRESYNC MUST implement and advertise support for
   the ENABLE [RFC5161] IMAP extension.  Also, the presence of the
   "QRESYNC" capability implies support for the CONDSTORE IMAP extension
   even if the "CONDSTORE" capability isn't advertised.  A server
   compliant with this specification is REQUIRED to support "ENABLE
   QRESYNC" and "ENABLE QRESYNC CONDSTORE" (which are "CONDSTORE
   enabling commands", see Section 3.1, and have identical results).
   Note that the order of parameters is not significant, but there is no
   requirement for a compliant server to support "ENABLE CONDSTORE" by
   itself.  The "ENABLE QRESYNC"/"ENABLE QRESYNC CONDSTORE" command also
   tells the server that it MUST start sending VANISHED responses (see

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   Section 3.2.10) instead of EXPUNGE responses for all mailboxes for
   which the server doesn't return the NOMODSEQ response code.  This
   change remains in effect until the connection is closed.

   A client making use of QRESYNC MUST issue "ENABLE QRESYNC" once it is
   authenticated.  A server MUST respond with a tagged BAD response if
   the QRESYNC parameter to the SELECT/EXAMINE command or the VANISHED
   UID FETCH modifier is specified and the client hasn't issued "ENABLE
   QRESYNC", or the server has not positively responded (in the current
   connection) to that command with the untagged ENABLED response
   containing QRESYNC.

3.2.4.  Additional Requirements on QRESYNC Servers

   Once a CONDSTORE enabling command is issued by the client, the server
   MUST automatically include both UID and mod-sequence data in all
   subsequent untagged FETCH responses (until the connection is closed),
   whether they were caused by a regular STORE/UID STORE, a STORE/UID
   STORE with an UNCHANGEDSINCE modifier, a FETCH/UID FETCH that
   implicitly set the \Seen flag, or an external agent.  Note that this
   rule doesn't affect untagged FETCH responses caused by a FETCH
   command that doesn't include UID and/or a MODSEQ FETCH data item (and
   doesn't implicitly set the \Seen flag) or UID FETCH without the
   MODSEQ FETCH data item.

3.2.5.  QRESYNC Parameter to SELECT/EXAMINE

   The Quick Resynchronization parameter to SELECT/EXAMINE commands has
   four arguments:

   o  the last known UIDVALIDITY,

   o  the last known modification sequence,

   o  the optional set of known UIDs, and

   o  an optional parenthesized list of known sequence ranges and their
      corresponding UIDs.

   A server MUST respond with a tagged BAD response if the Quick
   Resynchronization parameter to the SELECT/EXAMINE command is
   specified and the client hasn't issued "ENABLE QRESYNC" in the
   current connection, or the server has not positively responded to
   that command with the untagged ENABLED response containing QRESYNC.

   Before opening the specified mailbox, the server verifies all
   arguments for syntactic validity.  If any parameter is not
   syntactically valid, the server returns the tagged BAD response, and

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   the mailbox remains unselected.  Once the check is done, the server
   opens the mailbox as if no SELECT/EXAMINE parameters are specified
   (this is subject to the processing of other parameters as defined in
   other extensions).  In particular, this means that the server MUST
   send all untagged responses as specified in Sections 6.3.1 and 6.3.2
   of [RFC3501].

   After that, the server checks the UIDVALIDITY value provided by the
   client.  If the provided UIDVALIDITY doesn't match the UIDVALIDITY
   for the mailbox being opened, then the server MUST ignore the
   remaining parameters and behave as if no dynamic message data
   changed.  The client can discover this situation by comparing the
   UIDVALIDITY value returned by the server.  This behavior allows the
   client not to synchronize the mailbox or decide on the best
   synchronization strategy.

   Example: Attempting to resynchronize INBOX, but the provided
            UIDVALIDITY parameter doesn't match the current UIDVALIDITY
            value.

            C: A02 SELECT INBOX (QRESYNC (67890007 20050715194045000
                41,43:211,214:541))
            S: * 464 EXISTS
            S: * 3 RECENT
            S: * OK [UIDVALIDITY 3857529045] UIDVALIDITY
            S: * OK [UIDNEXT 550] Predicted next UID
            S: * OK [HIGHESTMODSEQ 90060128194045007] Highest mailbox
            mod-sequence
            S: * OK [UNSEEN 12] Message 12 is first unseen
            S: * FLAGS (\Answered \Flagged \Draft \Deleted \Seen)
            S: * OK [PERMANENTFLAGS (\Answered \Flagged \Draft
            \Deleted \Seen \*)] Permanent flags
            S: A02 OK [READ-WRITE] Sorry, UIDVALIDITY mismatch

   Remaining parameters are described in the following subsections.

3.2.5.1.  Modification Sequence and UID Parameters

   A server that doesn't support the persistent storage of mod-sequences
   for the mailbox MUST send an OK untagged response including the
   NOMODSEQ response code with every successful SELECT or EXAMINE
   command (see Section 3.1.2.2).  Such a server doesn't need to
   remember mod-sequences for expunged messages in the mailbox.  It MUST
   ignore the remaining parameters and behave as if no dynamic message
   data changed.

   If the provided UIDVALIDITY matches that of the selected mailbox, the
   server then checks the last known modification sequence.

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   The server sends the client any pending flag changes (using FETCH
   responses that MUST contain UIDs) and expunges those that have
   occurred in this mailbox since the provided modification sequence.

   If the list of known UIDs was also provided, the server should only
   report flag changes and expunges for the specified messages.  If the
   client did not provide the list of UIDs, the server acts as if the
   client has specified "1:<maxuid>", where <maxuid> is the mailbox's
   UIDNEXT value minus 1.  If the mailbox is empty and never had any
   messages in it, then lack of the list of UIDs is interpreted as an
   empty set of UIDs.

   Thus, the client can process just these pending events and need not
   perform a full resynchronization.  Without the message sequence
   number matching information, the result of this step is semantically
   equivalent to the client issuing:
   tag1 UID FETCH "known-uids" (FLAGS) (CHANGEDSINCE "mod-sequence-
   value" VANISHED)

   In particular, this means that all requirements specified in
   Section 3.2.6 apply.

   Example:

      C: A03 SELECT INBOX (QRESYNC (67890007
          90060115194045000 41:211,214:541))
      S: * OK [CLOSED]
      S: * 100 EXISTS
      S: * 11 RECENT
      S: * OK [UIDVALIDITY 67890007] UIDVALIDITY
      S: * OK [UIDNEXT 600] Predicted next UID
      S: * OK [HIGHESTMODSEQ 90060115205545359] Highest
          mailbox mod-sequence
      S: * OK [UNSEEN 7] There are some unseen
          messages in the mailbox
      S: * FLAGS (\Answered \Flagged \Draft \Deleted \Seen)
      S: * OK [PERMANENTFLAGS (\Answered \Flagged \Draft
          \Deleted \Seen \*)] Permanent flags
      S: * VANISHED (EARLIER) 41,43:116,118,120:211,214:540
      S: * 49 FETCH (UID 117 FLAGS (\Seen \Answered) MODSEQ
          (90060115194045001))
      S: * 50 FETCH (UID 119 FLAGS (\Draft $MDNSent) MODSEQ
          (90060115194045308))
      S: * 51 FETCH (UID 541 FLAGS (\Seen $Forwarded) MODSEQ
          (90060115194045001))
      S: A03 OK [READ-WRITE] mailbox selected

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   In the above example, flag information for UID 42 is not returned,
   presumably because its flags haven't changed since the MODSEQ
   90060115194045000.

3.2.5.2.  Message Sequence Match Data

   A client MAY provide a parenthesized list of a message sequence set
   and the corresponding UID sets.  Both MUST be provided in ascending
   order.  The server uses this data to restrict the range for which it
   provides expunged message information.

   Conceptually, the client provides a small sample of sequence numbers
   for which it knows the corresponding UIDs.  The server then compares
   each sequence number and UID pair the client provides with the
   current state of the mailbox.  If a pair matches, then the client
   knows of any expunges up to, and including, the message; thus, it
   will not include that range in the VANISHED response, even if the
   "mod-sequence-value" provided by the client is too old for the server
   to have data of when those messages were expunged.

   Thus, if the Nth message number in the first set in the list is 4,
   and the Nth UID in the second set in the list is 8, and the mailbox's
   fourth message has UID 8, then no UIDs equal to or less than 8 are
   present in the VANISHED response.  If the (N+1)th message number is
   12, and the (N+1)th UID is 24, and the (N+1)th message in the mailbox
   has UID 25, then the lowest UID included in the VANISHED response
   would be 9.

   In the following two examples, the server is unable to remember
   expunges at all, and only UIDs with messages divisible by three are
   present in the mailbox.  In the first example, the client does not
   use the fourth parameter; in the second, it provides it.  This
   example is somewhat extreme, but it shows that judicious usage of the
   sequence match data can save a substantial amount of bandwidth.

   Example:

      C: A04 SELECT INBOX (QRESYNC (67890007
          90060115194045000 1:29997))
      S: * 10003 EXISTS
      S: * 4 RECENT
      S: * OK [UIDVALIDITY 67890007] UIDVALIDITY
      S: * OK [UIDNEXT 30013] Predicted next UID
      S: * OK [HIGHESTMODSEQ 90060115205545359] Highest mailbox
          mod-sequence
      S: * OK [UNSEEN 7] There are some unseen messages in the mailbox
      S: * FLAGS (\Answered \Flagged \Draft \Deleted \Seen)

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      S: * OK [PERMANENTFLAGS (\Answered \Flagged \Draft
          \Deleted \Seen \*)] Permanent flags
      S: * VANISHED (EARLIER) 1:2,4:5,7:8,10:11,13:14,[...],
          29668:29669,29671:29996
      S: * 1 FETCH (UID 3 FLAGS (\Seen \Answered $Important) MODSEQ
          (90060115194045001))
      S: ...
      S: * 9889 FETCH (UID 29667 FLAGS (\Seen \Answered) MODSEQ
          (90060115194045027))
      S: * 9890 FETCH (UID 29670 FLAGS (\Draft $MDNSent) MODSEQ
          (90060115194045028))
      S: ...
      S: * 9999 FETCH (UID 29997 FLAGS (\Seen $Forwarded) MODSEQ
          (90060115194045031))
      S: A04 OK [READ-WRITE] mailbox selected

   Example:

      C: B04 SELECT INBOX (QRESYNC (67890007
         90060115194045000 1:29997 (5000,7500,9000,9990:9999 15000,
         22500,27000,29970,29973,29976,29979,29982,29985,29988,29991,
         29994,29997)))
      S: * 10003 EXISTS
      S: * 4 RECENT
      S: * OK [UIDVALIDITY 67890007] UIDVALIDITY
      S: * OK [UIDNEXT 30013] Predicted next UID
      S: * OK [HIGHESTMODSEQ 90060115205545359] Highest mailbox mod-
         sequence
      S: * OK [UNSEEN 7] There are some unseen messages in the mailbox
      S: * FLAGS (\Answered \Flagged \Draft \Deleted \Seen)
      S: * OK [PERMANENTFLAGS (\Answered \Flagged \Draft
         \Deleted \Seen \*)] Permanent flags
      S: * 1 FETCH (UID 3 FLAGS (\Seen \Answered $Important) MODSEQ
         (90060115194045001))
      S: ...
      S: * 9889 FETCH (UID 29667 FLAGS (\Seen \Answered) MODSEQ
         (90060115194045027))
      S: * 9890 FETCH (UID 29670 FLAGS (\Draft $MDNSent) MODSEQ
         (90060115194045028))
      S: ...
      S: * 9999 FETCH (UID 29997 FLAGS (\Seen $Forwarded) MODSEQ
         (90060115194045031))
      S: B04 OK [READ-WRITE] mailbox selected

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3.2.6.  VANISHED UID FETCH Modifier

   [RFC4466] has extended the syntax of the FETCH and UID FETCH commands
   to include an optional FETCH modifier.  This document defines a new
   UID FETCH modifier: VANISHED.

   Note that the VANISHED UID FETCH modifier is NOT allowed with a FETCH
   command.  The server MUST return a tagged BAD response if this
   response is specified as a modifier to the FETCH command.

   A server MUST respond with a tagged BAD response if the VANISHED UID
   FETCH modifier is specified and the client hasn't issued "ENABLE
   QRESYNC" in the current connection.

   The VANISHED UID FETCH modifier MUST only be specified together with
   the CHANGEDSINCE UID FETCH modifier.  If the VANISHED UID FETCH
   modifier is used without the CHANGEDSINCE UID FETCH modifier, the
   server MUST respond with a tagged BAD response.

   The VANISHED UID FETCH modifier instructs the server to report those
   messages from the UID set parameter that have been expunged and whose
   associated mod-sequence is larger than the specified mod-sequence.
   That is, the client requests to be informed of messages from the
   specified set that were expunged since the specified mod-sequence.
   Note that the mod-sequence(s) associated with these messages was
   updated when the messages were expunged (as described above).  The
   expunged messages are reported using the VANISHED (EARLIER) response
   as described in Section 3.2.10.1.  Any VANISHED (EARLIER) responses
   MUST be returned before any FETCH responses, otherwise the client
   might get confused about how message numbers map to UIDs.

   Note: A server that receives a mod-sequence smaller than <minmodseq>,
   where <minmodseq> is the value of the smallest expunged mod-sequence
   it remembers minus one, MUST behave as if it was requested to report
   all expunged messages from the provided UID set parameter.

   Example 1: Without the VANISHED UID FETCH modifier, a CONDSTORE-aware
   client needs to issue separate commands to learn of flag changes and
   expunged messages since the last synchronization:

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   C: s100 UID FETCH 300:500 (FLAGS) (CHANGEDSINCE 12345)
   S: * 1 FETCH (UID 404 MODSEQ (65402) FLAGS (\Seen))
   S: * 2 FETCH (UID 406 MODSEQ (75403) FLAGS (\Deleted))
   S: * 4 FETCH (UID 408 MODSEQ (29738) FLAGS ($NoJunk
       $AutoJunk $MDNSent))
   S: s100 OK FETCH completed
   C: s101 UID SEARCH 300:500
   S: * SEARCH 404 406 407 408 410 412
   S: s101 OK search completed

   Where 300 and 500 are the lowest and highest UIDs from the client's
   cache.  The second SEARCH response tells the client that the messages
   with UIDs 407, 410, and 412 are still present, but their flags
   haven't changed since the specified modification sequence.

   Using the VANISHED UID FETCH modifier, it is sufficient to issue only
   a single command:

   C: s100 UID FETCH 300:500 (FLAGS) (CHANGEDSINCE 12345
       VANISHED)
   S: * VANISHED (EARLIER) 300:310,405,411
   S: * 1 FETCH (UID 404 MODSEQ (65402) FLAGS (\Seen))
   S: * 2 FETCH (UID 406 MODSEQ (75403) FLAGS (\Deleted))
   S: * 4 FETCH (UID 408 MODSEQ (29738) FLAGS ($NoJunk
       $AutoJunk $MDNSent))
   S: s100 OK FETCH completed

3.2.7.  EXPUNGE Command

   Arguments: none

   Responses: untagged responses: EXPUNGE or VANISHED

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

   This section updates the definition of the EXPUNGE command described
   in Section 6.4.3 of [RFC3501].

   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, those messages that are removed are
   reported using a VANISHED response or EXPUNGE responses.

   If the server is capable of storing modification sequences for the
   selected mailbox, it MUST increment the per-mailbox mod-sequence if
   at least one message was permanently removed due to the execution of

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   the EXPUNGE command.  For each permanently removed message, the
   server MUST remember the incremented mod-sequence and corresponding
   UID.  If at least one message got expunged and QRESYNC was enabled,
   the server MUST send the updated per-mailbox modification sequence
   using the HIGHESTMODSEQ response code (see Section 3.1.2.1) in the
   tagged OK response.

      Example:    C: A202 EXPUNGE
                  S: * 3 EXPUNGE
                  S: * 3 EXPUNGE
                  S: * 5 EXPUNGE
                  S: * 8 EXPUNGE
                  S: A202 OK [HIGHESTMODSEQ 20010715194045319] expunged

   Note: In this example, the client hasn't enabled QRESYNC, so the
   server is still using untagged EXPUNGE responses.  Note that the
   presence of the HIGHESTMODSEQ response code is optional in this case.
   If the selected mailbox returned NOMODSEQ, the HIGHESTMODSEQ response
   code will be absent.  In this example, messages 3, 4, 7, and 11 had
   the \Deleted flag set.  The first "* 3 EXPUNGE" reports message #3 as
   expunged.  The second "* 3 EXPUNGE" reports message #4 as expunged
   (the message number was decremented due to the previous EXPUNGE
   response).  See the description of the EXPUNGE response in [RFC3501]
   for further explanation.

   Once the client enables QRESYNC, the server will always send VANISHED
   responses instead of EXPUNGE responses for mailboxes that support the
   storing of modification sequences, so the previous example might look
   like this:

      Example:    C: B202 EXPUNGE
                  S: * VANISHED 405,407,410,425
                  S: B202 OK [HIGHESTMODSEQ 20010715194045319] expunged

   Here, messages with message numbers 3, 4, 7, and 11 have respective
   UIDs 405, 407, 410, and 425.

3.2.8.  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

   This section updates the definition of the CLOSE command described in
   Section 6.4.2 of [RFC3501].

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   The CLOSE command permanently removes all messages that have the
   \Deleted flag set from the currently selected mailbox and returns to
   the authenticated state from the selected state.  No untagged EXPUNGE
   (or VANISHED) responses are sent.

   If the server is capable of storing modification sequences for the
   selected mailbox, it MUST increment the per-mailbox mod-sequence if
   at least one message was permanently removed due to the execution of
   the CLOSE command.  For each permanently removed message, the server
   MUST remember the incremented mod-sequence and corresponding UID.
   The server MUST NOT send the updated per-mailbox modification
   sequence using the HIGHESTMODSEQ response code (see Section 3.1.2.1)
   in the tagged OK response, as this might cause loss of
   synchronization on the client.

      Example:    C: A202 CLOSE
                  S: A202 OK done

3.2.9.  UID EXPUNGE Command

   Arguments: message set

   Responses: untagged responses: EXPUNGE or VANISHED

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

   This section updates the definition of the UID EXPUNGE command
   described in Section 2.1 of [UIDPLUS], in the presence of QRESYNC.
   Servers that implement both [UIDPLUS] and QRESYNC extensions must
   implement UID EXPUNGE as described in this section.

   The UID EXPUNGE command permanently removes from the currently
   selected mailbox all messages that have both the \Deleted flag set
   and a UID that is included in the specified message set.  If a
   message either does not have the \Deleted flag set or has a UID that
   is not included in the specified message set, it is not affected.

   This command is particularly useful for disconnected mode clients.
   By using UID EXPUNGE instead of EXPUNGE when resynchronizing with the
   server, the client can avoid inadvertently removing any messages that
   have been marked as \Deleted by other clients between the time that
   the client was last connected and the time the client resynchronizes.

   Before returning an OK to the client, those messages that are removed
   are reported using a VANISHED response or EXPUNGE responses.

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   If the server is capable of storing modification sequences for the
   selected mailbox, it MUST increment the per-mailbox mod-sequence if
   at least one message was permanently removed due to the execution of
   the UID EXPUNGE command.  For each permanently removed message, the
   server MUST remember the incremented mod-sequence and corresponding
   UID.  If at least one message got expunged and QRESYNC was enabled,
   the server MUST send the updated per-mailbox modification sequence
   using the HIGHESTMODSEQ response code (see Section 3.1.2.1) in the
   tagged OK response.

   Example:    C: . UID EXPUNGE 3000:3002
               S: * 3 EXPUNGE
               S: * 3 EXPUNGE
               S: * 3 EXPUNGE
               S: . OK [HIGHESTMODSEQ 20010715194045319] Ok

   Note: In this example, the client hasn't enabled QRESYNC, so the
   server is still using untagged EXPUNGE responses instead of VANISHED
   responses.  Note that the presence of the HIGHESTMODSEQ response code
   is optional.  If the selected mailbox returned NOMODSEQ, the
   HIGHESTMODSEQ response code will be absent.  In this example, at
   least messages with message numbers 3, 4, and 5 (UIDs 3000 to 3002)
   had the \Deleted flag set.  The first "* 3 EXPUNGE" reports message
   #3 as expunged.  The second "* 3 EXPUNGE" reports message #4 as
   expunged (the message number was decremented due to the previous
   EXPUNGE response).  See the description of the EXPUNGE response in
   [RFC3501] for further explanation.

3.2.10.  VANISHED Response

   The VANISHED response reports that the specified UIDs have been
   permanently removed from the mailbox.  This response is similar to
   the EXPUNGE response [RFC3501]; however, it can return information
   about multiple messages, and it returns UIDs instead of message
   numbers.  The first benefit saves bandwidth, while the second is more
   convenient for clients that only use UIDs to access the IMAP server.

   The VANISHED response has the same restrictions on when it can be
   sent as does the EXPUNGE response (see below).  Once a client has
   issued "ENABLE QRESYNC" (and the server has positively responded to
   that command with the untagged ENABLED response containing QRESYNC),
   the server MUST use the VANISHED response without the EARLIER tag
   instead of the EXPUNGE response for all mailboxes that don't return
   NOMODSEQ when selected.  The server continues using VANISHED in lieu
   of EXPUNGE for the duration of the connection.  In particular, this
   affects the EXPUNGE [RFC3501] and UID EXPUNGE [UIDPLUS] commands, as
   well as messages expunged in other connections.  Such a VANISHED
   response MUST NOT contain the EARLIER tag.

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   The VANISHED response has two forms.  The first form contains the
   EARLIER tag, which signifies that the response was caused by a UID
   FETCH (VANISHED) or a SELECT/EXAMINE (QRESYNC) command.  The second
   form doesn't contain the EARLIER tag and is used for announcing
   message removals within an already selected mailbox.

   Because clients handle the two different forms of the VANISHED
   response differently, servers MUST NOT combine them.  Messages are
   reported in VANISHED responses with or without the EARLIER tag, as
   appropriate to the cause, and, if necessary, two VANISHED responses
   are sent (one with EARLIER and one without).

3.2.10.1.  VANISHED (EARLIER) Response

   Contents:  an EARLIER tag

              list of UIDs

   The VANISHED (EARLIER) response is caused by a UID FETCH (VANISHED)
   or a SELECT/EXAMINE (QRESYNC) command.  This response is sent if the
   UID set parameter to the UID FETCH (VANISHED) command includes UIDs
   of messages that are no longer in the mailbox.  When the client sees
   a VANISHED EARLIER response, it MUST NOT decrement message sequence
   numbers for each successive message in the mailbox.

3.2.10.2.  VANISHED Response without the (EARLIER) Tag

   Contents:  list of UIDs

   Once a client has issued "ENABLE QRESYNC" (and the server has
   positively responded to that command with the untagged ENABLED
   response containing QRESYNC), the server MUST use the VANISHED
   response without the EARLIER tag instead of the EXPUNGE response for
   all mailboxes that don't return NOMODSEQ when selected.  The server
   continues using VANISHED in lieu of EXPUNGE for the duration of the
   connection.  In particular, this affects the EXPUNGE [RFC3501] and
   UID EXPUNGE [UIDPLUS] commands, as well as messages expunged in other
   connections.  Such a VANISHED response MUST NOT contain the EARLIER
   tag.

   Unlike VANISHED (EARLIER), this response also decrements the number
   of messages in the mailbox and adjusts the message sequence numbers
   for the messages remaining in the mailbox to account for the expunged
   messages.  Because of this housekeeping, it is not necessary for the
   server to send an EXISTS response to report the new message count.
   See the example at the end of this section.

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   A VANISHED response without the EARLIER tag MUST refer only to
   messages that are visible to the client in the current session at the
   time the VANISHED response is sent.  That is, servers MUST NOT send
   UIDs for previously expunged messages or messages that were not
   announced to the client via EXISTS.  This means that each UID listed
   in a VANISHED response results in the client decrementing the message
   count by one.  This is required to prevent a possible race condition
   where new arrivals for which the UID is not yet known by the client
   are immediately expunged.

   A VANISHED 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 the 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.  A VANISHED response MAY be sent
   during a UID command.  However, the VANISHED response MUST NOT be
   sent during a UID SEARCH command that contains message numbers in the
   search criteria.

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

   Example: Let's assume that there is the following mapping between
   message numbers and UIDs in the currently selected mailbox (here "D"
   marks messages with the \Deleted flag set, and "x" represents UIDs,
   which are not relevant for the example):

   Message numbers:   1    2    3    4    5  6   7  8  9 10  11
   UIDs:              x  504  505  507  508  x 510  x  x  x 625
   \Deleted messages:           D    D           D            D

   In the presence of the extension defined in this document:

   C: A202 EXPUNGE
   S: * VANISHED 505,507,510,625
   S: A202 OK EXPUNGE completed

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   Without the QRESYNC extension, the same example might look like:

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

   (Continuing from the previous example.)  If subsequently messages
   with UIDs 504 and 508 got marked as \Deleted:

   C: A210 EXPUNGE
   S: * VANISHED 504,508
   S: A210 OK EXPUNGE completed

   For Example, the last VANISHED response only contains UIDs of
   messages expunged since the previous VANISHED response.

   To illustrate the difference between VANISHED and VANISHED (EARLIER),
   suppose the mailbox contains UIDs 2 and 4.  Any of the following
   responses would constitute a broken server implementation:

   S: * VANISHED 1
   S: * VANISHED 3
   S: * VANISHED 5

   However, any of these UIDs can easily be referenced by the VANISHED
   (EARLIER) response.

3.2.11.  CLOSED Response Code

   The CLOSED response code has no parameters.  A server implementing
   the extension defined in this document MUST return the CLOSED
   response code when the currently selected mailbox is closed
   implicitly using the SELECT/EXAMINE command on another mailbox.  The
   CLOSED response code serves as a boundary between responses for the
   previously opened mailbox (which was closed) and the newly selected
   mailbox; all responses before the CLOSED response code relate to the
   mailbox that was closed, and all subsequent responses relate to the
   newly opened mailbox.

   A server that advertises "QRESYNC" or "CONDSTORE" in the capability
   string must return the CLOSED response code in this case, whether or
   not a CONDSTORE enabling command was issued.

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   There is no need to return the CLOSED response code on completion of
   the CLOSE or the UNSELECT [UNSELECT] command (or similar), whose
   purpose is to close the currently selected mailbox without opening a
   new one.

4.  Long Command Lines (Update to RFC 2683)

   While [RFC3501] doesn't specify a specific line-length limit, several
   server implementations chose to implement the recommended line-length
   limit suggested in Section 3.2.1.5 of [RFC2683] in order to protect
   from Denial-of-Service attacks.  When the line-length limit is
   exceeded, such servers return a BAD response (as required by
   [RFC3501] in case of a syntactic error) and may even close the
   connection.  Clients that support CONDSTORE/QRESYNC extensions can
   trigger this limit by sending a long UID sequence (previously
   returned by the server) in an extended SELECT or FETCH command.

   This document updates recommended line-length limits specified in
   Section 3.2.1.5 of [RFC2683].  While the advice in the first
   paragraph of that section still applies (use compact message/UID set
   representations), the 1000-octet limit suggested in the second
   paragraph turns out to be quite problematic when the CONDSTORE and/or
   QRESYNC extension is used.

   The updated recommendation is as follows: a client should limit the
   length of the command lines it generates to approximately 8192 octets
   (including all quoted strings but not including literals).  If the
   client is unable to group things into ranges so that the command line
   is within that length, it should split the request into multiple
   commands.  The client should use literals instead of long quoted
   strings in order to keep the command length down.

5.  QRESYNC Server Implementation Considerations

   This section describes a minimalist implementation, a moderate
   implementation, and an example of a full implementation.

5.1.  Server Implementations That Don't Store Extra State

   Strictly speaking, a server implementation that doesn't remember mod-
   sequences associated with expunged messages can be considered
   compliant with this specification.  Such implementations return all
   expunged messages specified in the UID set of the UID FETCH
   (VANISHED) command every time, without paying attention to the
   specified CHANGEDSINCE mod-sequence.  Such implementations are
   discouraged as they can end up returning VANISHED responses that are
   bigger than the result of a UID SEARCH command for the same UID set.

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   A client can substantially reduce the size of VANISHED responses by
   providing the server with message sequence match data (see
   Section 3.2.5.2).  This is especially effective in the typical case
   where no messages have been expunged, or all expunges were toward the
   end of the mailbox.

5.2.  Server Implementations Storing Minimal State

   A server that stores the HIGHESTMODSEQ value at the time of the last
   EXPUNGE can omit the VANISHED response when a client provides a
   MODSEQ value that is equal to or higher than that HIGHESTMODSEQ value
   because there have been no messages expunged during the time period
   the client is concerned about.

   A client providing message sequence match data can reduce the scope
   as above.  In the case where there have been no expunges, the server
   can ignore this data.

5.3.  Additional State Required on the Server

   When compared to the CONDSTORE extension, QRESYNC requires servers to
   store an additional state associated with expunged messages.  Note
   that implementations are not required to store this state in
   persistent storage; however, use of persistent storage is advisable.

   One possible way to correctly implement QRESYNC is to store a queue
   of <UID set, mod-sequence> pairs. <UID set> can be represented as a
   sequence of <min UID, max UID> pairs.

   When messages are expunged, one or more entries are added to the
   queue tail.

   When the server receives a request to return messages expunged since
   a given mod-sequence, it will search the queue from the tail (i.e.,
   going from the highest expunged mod-sequence to the lowest) until it
   sees the first record with a mod-sequence less than or equal to the
   given mod-sequence or it reaches the head of the queue.

   Note that indefinitely storing information about expunged messages
   can cause storage and related problems for an implementation.  In the
   worst case, this could result in almost 64 GB of storage for each
   IMAP mailbox.  For example, consider an implementation that stores
   <min UID, max UID, mod-sequence> triples for each range of messages
   expunged at the same time.  Each triple requires 16 octets: 4 octets
   for each of the two UIDs and 8 octets for the mod-sequence.  Assume
   that there is a mailbox containing a single message with a UID of
   2**32-1 (the maximum possible UID value), where messages had
   previously existed with UIDs starting at 1 and have been expunged one

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   at a time.  For this mailbox alone, storage is required for the
   triples <1, 1, modseq1>, <2, 2, modseq2>, ..., <2**32-2, 2**32-2,
   modseq4294967294>.

   Hence, implementations are encouraged to adopt strategies to protect
   against such storage problems, such as limiting the size of the queue
   used to store mod-sequences for expunged messages and "expiring"
   older records when this limit is reached.  When the selected
   implementation-specific queue limit is reached, the oldest record(s)
   is deleted from the queue (note that such records are located at the
   queue head).  For all such "expired" records, the server needs to
   store a single mod-sequence, which is the highest mod-sequence for
   all "expired" expunged messages.

   If the client provides the message sequence match data, this can
   heavily reduce the data cost of sending a complete set of missing
   UIDs; thus, it reduces the problems for clients if a server is unable
   to persist much of this queue.  If the queue contains data back to
   the requested mod-sequence, this data can be ignored.

   Also, note that if the UIDVALIDITY of the mailbox changes or if the
   mailbox is deleted, then any state associated with expunged messages
   doesn't need to be preserved and SHOULD be deleted.

6.  Updated Synchronization Sequence

   This section updates the description of optimized synchronization in
   Section 6.1 of [IMAP-DISC], in the presence of QRESYNC.

   An advanced disconnected mail client SHOULD use the QRESYNC extension
   when it is supported by the server and SHOULD use CONDSTORE if it is
   supported and QRESYNC is not.  The client uses the value from the
   HIGHESTMODSEQ OK response code received on the mailbox opening to
   determine if it needs to resynchronize.  Once the synchronization is
   complete, it MUST cache the received value (unless the mailbox
   UIDVALIDITY value has changed; see below).  The client MUST update
   its copy of the HIGHESTMODSEQ value whenever the server sends a
   subsequent HIGHESTMODSEQ OK response code.

   After completing a full synchronization, the client MUST also take
   note of any unsolicited MODSEQ FETCH data items and HIGHESTMODSEQ
   response codes received from the server.  Whenever the client
   receives a tagged response to a command, it checks the received
   unsolicited responses to calculate the new HIGHESTMODSEQ value.  If
   the HIGHESTMODSEQ response code is received, the client MUST use it
   even if it has seen higher mod-sequences.  Otherwise, the client
   calculates the highest value among all MODSEQ FETCH data items

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   received since the last tagged response.  If this value is bigger
   than the client's copy of the HIGHESTMODSEQ value, then the client
   MUST use this value as its new HIGHESTMODSEQ value.

   Example:

   C: A150 STORE 1:2 (UNCHANGEDSINCE 96) +FLAGS.SILENT \Seen
   S: * 1 FETCH (UID 6 MODSEQ (103))
   S: * 2 FETCH (UID 7 MODSEQ (101))
   S: * OK [HIGHESTMODSEQ 99] VANISHED reply with MODSEQ 100 is delayed
   S: A150 OK [MODIFIED 3] done

   C: A151 STORE 3 +FLAGS.SILENT \Seen
   S: * 3 FETCH (UID 8 MODSEQ (104))
   S: A151 OK [HIGHESTMODSEQ 99] Still delaying VANISHED

   C: A152 NOOP
   S: * VANISHED 8
   S: A153 OK [HIGHESTMODSEQ 104] done

   Note: It is not safe to update the client's copy of the HIGHESTMODSEQ
   value with a MODSEQ FETCH data item value as soon as it is received
   because servers are not required to send MODSEQ FETCH data items in
   increasing mod-sequence order.  Some commands may also delay EXPUNGE
   (or VANISHED) replies with smaller mod-sequences.  These can lead to
   the client missing some changes in case of connectivity loss.

   When opening the mailbox for synchronization, the client uses the
   QRESYNC parameter to the SELECT/EXAMINE command.  The QRESYNC
   parameter is followed by the UIDVALIDITY and mailbox HIGHESTMODSEQ
   values, as known to the client.  It can be optionally followed by the
   set of UIDs, for example, if the client is only interested in partial
   synchronization of the mailbox.  The client may also transmit a list
   containing its knowledge of message numbers.

   If the SELECT/EXAMINE command is successful, the client compares
   UIDVALIDITY as described in step d-1 in Section 3 of the [IMAP-DISC].
   If the cached UIDVALIDITY value matches the one returned by the
   server and the server also returns the HIGHESTMODSEQ response code,
   then the server reports expunged messages and returns flag changes
   for all messages specified by the client in the UID set parameter (or
   for all messages in the mailbox, if the client omitted the UID set
   parameter).  At this point, the client is synchronized, except for
   maybe the new messages.

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   If upon a successful SELECT/EXAMINE (QRESYNC) command the client
   receives a NOMODSEQ OK untagged response (instead of the
   HIGHESTMODSEQ response code), it MUST remove the last known
   HIGHESTMODSEQ value from its cache and follow the more general
   instructions in Section 3 of the [IMAP-DISC].

   At this point, the client is in sync with the server regarding old
   messages.  This client can now fetch information about new messages
   (if requested by the user).

   Step d ("Server-to-client synchronization") in Section 6.1 of
   [IMAP-DISC] in the presence of the QRESYNC & CONDSTORE extensions is
   amended as follows:

   d) "Server-to-client synchronization" -- for each mailbox that
      requires synchronization, do the following:

   1a) Check the mailbox UIDVALIDITY (see Section 4.1 of [IMAP-DISC] for
       more details) after issuing the SELECT/EXAMINE (QRESYNC) command.

       If the UIDVALIDITY value returned by the server differs, the
       client MUST:

       *  empty the local cache of that mailbox;

       *  "forget" the cached HIGHESTMODSEQ value for the mailbox; and

       *  remove any pending "actions" that refer to UIDs in that
          mailbox.  Note, this doesn't affect actions performed on
          client-generated fake UIDs (see Section 5 of the [IMAP-DISC]).

   1b) This step is no longer required.

   2)  Fetch the current "descriptors".

       I) Discover new messages.

   3)  Fetch the bodies of any "interesting" messages that the client
       doesn't already have.

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   Example: The UIDVALIDITY value is the same, but the HIGHESTMODSEQ
            value has changed on the server while the client was
            offline:

    C: A142 SELECT INBOX (QRESYNC (3857529045 20010715194032001 1:198))
    S: * 172 EXISTS
    S: * 1 RECENT
    S: * OK [UNSEEN 12] Message 12 is first unseen
    S: * OK [UIDVALIDITY 3857529045] UIDs valid
    S: * OK [UIDNEXT 201] Predicted next UID
    S: * FLAGS (\Answered \Flagged \Deleted \Seen \Draft)
    S: * OK [PERMANENTFLAGS (\Deleted \Seen \*)] Limited
    S: * OK [HIGHESTMODSEQ 20010715194045007] Highest
          mailbox mod-sequence
    S: * VANISHED (EARLIER) 1:5,7:8,10:15
    S: * 2 FETCH (UID 6 MODSEQ (20010715205008000)
        FLAGS (\Deleted))
    S: * 5 FETCH (UID 9 MODSEQ (20010715195517000)
        FLAGS ($NoJunk $AutoJunk $MDNSent))
       ...
    S: A142 OK [READ-WRITE] SELECT completed

7.  Formal Syntax

   The following syntax specification uses the Augmented Backus-Naur
   Form (ABNF) notation as specified in [RFC5234].

   Non-terminals referenced but not defined below are as defined by
   [RFC5234], [RFC3501], or [RFC4466].

   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.

   capability          =/ "CONDSTORE" / "QRESYNC"

   status-att          =/ "HIGHESTMODSEQ"
                          ;; Extends non-terminal defined in [RFC3501].

   status-att-val      =/ "HIGHESTMODSEQ" SP mod-sequence-valzer
                          ;; Extends non-terminal defined in [RFC4466].
                          ;; Value 0 denotes that the mailbox doesn't
                          ;; support persistent mod-sequences
                          ;; as described in Section 3.1.2.2.

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   store-modifier      =/ "UNCHANGEDSINCE" SP mod-sequence-valzer
                          ;; Only a single "UNCHANGEDSINCE" may be
                          ;; specified in a STORE operation.

   fetch-modifier      =/ chgsince-fetch-mod
                          ;; Conforms to the generic "fetch-modifier"
                          ;; syntax defined in [RFC4466].

   chgsince-fetch-mod  = "CHANGEDSINCE" SP mod-sequence-value
                          ;; CHANGEDSINCE FETCH modifier conforms to
                          ;; the fetch-modifier syntax.

   fetch-att           =/ fetch-mod-sequence
                          ;; Modifies original IMAP4 fetch-att.

   fetch-mod-sequence  = "MODSEQ"

   fetch-mod-resp      = "MODSEQ" SP "(" permsg-modsequence ")"

   msg-att-dynamic     =/ fetch-mod-resp

   search-key          =/ search-modsequence
                          ;; Modifies original IMAP4 search-key.
                          ;;
                          ;; This change applies to all commands
                          ;; referencing this non-terminal -- in
                          ;; particular, SEARCH, SORT, and THREAD.

   search-modsequence  = "MODSEQ" [search-modseq-ext] SP
                         mod-sequence-valzer

   search-modseq-ext   = SP entry-name SP entry-type-req

   resp-text-code      =/ "HIGHESTMODSEQ" SP mod-sequence-value /
                          "NOMODSEQ" /
                          "MODIFIED" SP sequence-set

   entry-name          = entry-flag-name

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   entry-flag-name     = DQUOTE "/flags/" attr-flag DQUOTE
                          ;; Each system or user-defined flag <flag>
                          ;; is mapped to "/flags/<flag>".
                          ;;
                          ;; <entry-flag-name> follows the escape rules
                          ;; used by "quoted" string as described in
                          ;; Section 4.3 of [RFC3501]; e.g., for the
                          ;; flag \Seen, the corresponding <entry-name>
                          ;; is "/flags/\\seen", and for the flag
                          ;; $MDNSent, the corresponding <entry-name>
                          ;; is "/flags/$mdnsent".

   entry-type-resp     = "priv" / "shared"
                          ;; Metadata item type.

   entry-type-req      = entry-type-resp / "all"
                          ;; Perform SEARCH operation on a private
                          ;; metadata item, shared metadata item,
                          ;; or both.

   permsg-modsequence  = mod-sequence-value
                          ;; Per-message mod-sequence.

   mod-sequence-value  = 1*DIGIT
                          ;; Positive unsigned 63-bit integer
                          ;; (mod-sequence)
                          ;; (1 <= n <= 9,223,372,036,854,775,807).

   mod-sequence-valzer = "0" / mod-sequence-value

   search-sort-mod-seq = "(" "MODSEQ" SP mod-sequence-value ")"

   select-param        =/ condstore-param
                          ;; Conforms to the generic "select-param"
                          ;; non-terminal syntax defined in [RFC4466].

   condstore-param     = "CONDSTORE"

   mailbox-data        =/ "SEARCH" [1*(SP nz-number) SP
                          search-sort-mod-seq]

   sort-data           = "SORT" [1*(SP nz-number) SP
                          search-sort-mod-seq]
                          ; Updates the SORT response from RFC 5256.

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

   attr-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 standards or Standards Track
                          ;; revisions of [RFC3501].

   attr-flag-keyword   = atom

   select-param        =/  "QRESYNC" SP "(" uidvalidity SP
                       mod-sequence-value [SP known-uids]
                       [SP seq-match-data] ")"
                       ;; Conforms to the generic select-param
                       ;; syntax defined in [RFC4466].

   seq-match-data      =  "(" known-sequence-set SP known-uid-set ")"

   uidvalidity         =  nz-number

   known-uids          =  sequence-set
                       ;; Sequence of UIDs; "*" is not allowed.

   known-sequence-set  =  sequence-set
                       ;; Set of message numbers corresponding to
                       ;; the UIDs in known-uid-set, in ascending order.
                       ;; * is not allowed.

   known-uid-set       =  sequence-set
                       ;; Set of UIDs corresponding to the messages in
                       ;; known-sequence-set, in ascending order.
                       ;; * is not allowed.

   message-data        =/ expunged-resp

   expunged-resp       =  "VANISHED" [SP "(EARLIER)"] SP known-uids

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   rexpunges-fetch-mod =  "VANISHED"
                       ;; VANISHED UID FETCH modifier conforms
                       ;; to the fetch-modifier syntax
                       ;; defined in [RFC4466].  It is only
                       ;; allowed in the UID FETCH command.

   resp-text-code      =/ "CLOSED"

8.  Security Considerations

   As always, it is important to thoroughly test clients and servers
   implementing QRESYNC, as it changes how the server reports expunged
   messages to the client.

   It is believed that the CONDSTORE or the QRESYNC extensions don't
   raise any new security concerns that are not already discussed in
   [RFC3501].  However, the availability of CONDSTORE may make it
   possible for IMAP4 to be used in critical applications it could not
   be used for previously, making correct IMAP server implementation and
   operation even more important.

9.  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/imap-capabilities

   This document defines the CONDSTORE and QRESYNC IMAP capabilities.
   IANA has updated references for both extensions to point to this
   document.

10.  References

10.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC2683]  Leiba, B., "IMAP4 Implementation Recommendations", RFC
              2683, September 1999.

   [RFC3501]  Crispin, M., "INTERNET MESSAGE ACCESS PROTOCOL - VERSION
              4rev1", RFC 3501, March 2003.

   [RFC4466]  Melnikov, A. and C. Daboo, "Collected Extensions to IMAP4
              ABNF", RFC 4466, April 2006.

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   [RFC5161]  Gulbrandsen, A. and A. Melnikov, "The IMAP ENABLE
              Extension", RFC 5161, March 2008.

   [RFC5234]  Crocker, D. and P. Overell, "Augmented BNF for Syntax
              Specifications: ABNF", STD 68, RFC 5234, January 2008.

   [RFC5256]  Crispin, M. and K. Murchison, "Internet Message Access
              Protocol - SORT and THREAD Extensions", RFC 5256, June
              2008.

   [RFC5464]  Daboo, C., "The IMAP METADATA Extension", RFC 5464,
              February 2009.

   [UIDPLUS]  Crispin, M., "Internet Message Access Protocol (IMAP) -
              UIDPLUS extension", RFC 4315, December 2005.

10.2.  Informative References

   [IMAP-DISC]
              Melnikov, A., Ed., "Synchronization Operations For
              Disconnected Imap4 Clients", RFC 4549, June 2006.

   [NTP]      Mills, D., Martin, J., Burbank, J., and W. Kasch, "Network
              Time Protocol Version 4: Protocol and Algorithms
              Specification", RFC 5905, June 2010.

   [RFC2180]  Gahrns, M., "IMAP4 Multi-Accessed Mailbox Practice", RFC
              2180, July 1997.

   [RFC4314]  Melnikov, A., "IMAP4 Access Control List (ACL) Extension",
              RFC 4314, December 2005.

   [RFC4731]  Melnikov, A. and D. Cridland, "IMAP4 Extension to SEARCH
              Command for Controlling What Kind of Information Is
              Returned", RFC 4731, November 2006.

   [RFC5257]  Daboo, C. and R. Gellens, "Internet Message Access
              Protocol - ANNOTATE Extension", RFC 5257, June 2008.

   [RFC5267]  Cridland, D. and C. King, "Contexts for IMAP4", RFC 5267,
              July 2008.

   [RFC6851]  Gulbrandsen, A. and N. Freed, "Internet Message Access
              Protocol (IMAP) - MOVE Extension", RFC 6851, January 2013.

   [UNSELECT] Melnikov, A., "Internet Message Access Protocol (IMAP)
              UNSELECT command", RFC 3691, February 2004.

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Appendix A.  Changes since RFC 4551

   Changed mod-sequences to be unsigned 63-bit values (instead of
   unsigned 64-bit values).

   Fixed the following errata, as posted on <http://www.rfc-editor.org>:

   o  Errata ID 3401 ("several typos in UNCHANGEDSINCE spelling")
   o  Errata ID 3506 ("invalid ABNF for the MODIFIED response code")
   o  Errata ID 3509 ("correction to an example")

   Clarified that the returning of HIGHESTMODSEQ/NOMODSEQ response codes
   is only required once a CONDSTORE enabling command is issued.

   Clarified that if multiple mod-sequences (for different metadata
   items) are associated with a message, then all of them affecting a
   particular STORE UNCHANGEDSINCE must be checked.

   Updated references.

   Made editorial corrections.

Appendix B.  Changes since RFC 5162

   Changed mod-sequences to be unsigned 63-bit values (instead of
   unsigned 64-bit values).

   Addressed the following errata, as posted on
   <http://www.rfc-editor.org>:

   o  Errata ID 1365 ("clarified that QRESYNC is only enabled when
      ENABLED QRESYNC is returned")
   o  Errata ID 1807 ("unsolicited FETCH responses must include UID
      fetch response item")
   o  Errata ID 1808 ("HIGHESTMODSEQ response code must not be returned
      for CLOSE")
   o  Errata ID 1809 ("clarify how updated mailbox mod-sequence is
      calculated")
   o  Errata ID 1810 ("server must send untagged events to client in a
      way that client doesn't lose any changes in case of connectivity
      loss")
   o  Errata ID 3322 ("VANISHED responses must not reference non-
      existing UIDs")

   Clarified that ENABLE QRESYNC CONDSTORE and ENABLE CONDSTORE QRESYNC
   are equivalent.

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   Changed the requirement to return VANISHED from SHOULD to MUST as per
   the mailing list discussion.  The only exception is for mailboxes
   that return the NOMODSEQ response code when they are selected.

   Specified that IMAP SETMETADATA changes update per-mailbox
   HIGHESTMODSEQ.

   Clarified that per-message annotations are also considered
   "metadata".

   Fixed some examples to report data that match requirements specified
   in the document.

   Clarified some text and made some requirements normative.  Also,
   corrected a couple of SHOULDs to be MUSTs.

   Updated references.

   Made editorial corrections.

Appendix C.  Acknowledgements

   Thank you to Steve Hole for co-editing RFC 4551.

   In this revision of the document, the authors also acknowledge the
   feedback provided by Timo Sirainen, Jan Kundrat, Pete Maclean, Barry
   Leiba, Eliot Lear, Chris Newman, Claudio Allocchio, Michael Slusarz,
   Bron Gondwana, Arnt Gulbrandsen, David Black, Hoa V. DINH, and Nick
   Hudson.

   Mark Crispin contributed to RFCs 4551 and 5162 that this document is
   replacing, and much of his contribution remains in this merged
   document.

   See also the list of people who contributed to RFC 4551, which this
   document obsoletes.

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Authors' Addresses

   Alexey Melnikov
   Isode Ltd
   5 Castle Business Village
   36 Station Road
   Hampton, Middlesex  TW12 2BX
   UK

   EMail: Alexey.Melnikov@isode.com


   Dave Cridland
   Surevine Ltd
   PO Box 1136
   Guildford, Surrey  GU1 9ND
   UK

   EMail: dave.cridland@surevine.com