path: root/doc/rfc/rfc7162.txt

Internet Engineering Task Force (IETF)                       A. Melnikov
Request for Comments: 7162                                     Isode Ltd
Obsoletes: 4551, 5162                                        D. Cridland
Updates: 2683                                               Surevine Ltd
Category: Standards Track                                       May 2014
ISSN: 2070-1721


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

Abstract

   Often, multiple IMAP (RFC 3501) clients need to coordinate changes to
   a common IMAP mailbox.  Examples include different clients working on
   behalf of the same user and multiple users accessing shared
   mailboxes.  These clients need a mechanism to efficiently synchronize
   state changes for messages within the mailbox.

   Initially defined in RFC 4551, the Conditional Store facility
   provides a protected update mechanism for message state information
   and a mechanism for requesting only changes to the message state.
   This memo updates that mechanism and obsoletes RFC 4551, based on
   operational experience.

   This document additionally updates another IMAP extension, Quick
   Resynchronization, which builds on the Conditional STORE extension to
   provide an IMAP client the ability to fully resynchronize a mailbox
   as part of the SELECT/EXAMINE command, without the need for
   additional server-side state or client round trips.  Hence, this memo
   obsoletes RFC 5162.

   Finally, this document also updates the line-length recommendation in
   Section 3.2.1.5 of RFC 2683.

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   http://www.rfc-editor.org/info/rfc7162.



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Copyright Notice

   Copyright (c) 2014 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
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   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.

























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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   4
   2.  Requirements Notation . . . . . . . . . . . . . . . . . . . .   5
   3.  IMAP Protocol Changes . . . . . . . . . . . . . . . . . . . .   5
     3.1.  CONDSTORE Extension . . . . . . . . . . . . . . . . . . .   5
       3.1.1.  Advertising Support for CONDSTORE . . . . . . . . . .   8
       3.1.2.  New OK Untagged Responses for SELECT and EXAMINE  . .   8
       3.1.3.  STORE and UID STORE Commands  . . . . . . . . . . . .  10
       3.1.4.  FETCH and UID FETCH Commands  . . . . . . . . . . . .  16
       3.1.5.  MODSEQ Search Criterion in SEARCH . . . . . . . . . .  19
       3.1.6.  Modified SEARCH Untagged Response . . . . . . . . . .  20
       3.1.7.  HIGHESTMODSEQ Status Data Items . . . . . . . . . . .  21
       3.1.8.  CONDSTORE Parameter to SELECT and EXAMINE . . . . . .  21
       3.1.9.  Interaction with IMAP SORT and THREAD Extensions  . .  22
       3.1.10. Interaction with IMAP ESORT and ESEARCH Extensions  .  22
       3.1.11. Additional Quality-of-Implementation Issues . . . . .  23
       3.1.12. CONDSTORE Server Implementation Considerations  . . .  23
     3.2.  QRESYNC Extension . . . . . . . . . . . . . . . . . . . .  24
       3.2.1.  Impact on CONDSTORE-only Clients  . . . . . . . . . .  25
       3.2.2.  Advertising Support for QRESYNC . . . . . . . . . . .  25
       3.2.3.  Use of ENABLE . . . . . . . . . . . . . . . . . . . .  25
       3.2.4.  Additional Requirements on QRESYNC Servers  . . . . .  26
       3.2.5.  QRESYNC Parameter to SELECT/EXAMINE . . . . . . . . .  26
       3.2.6.  VANISHED UID FETCH Modifier . . . . . . . . . . . . .  31
       3.2.7.  EXPUNGE Command . . . . . . . . . . . . . . . . . . .  32
       3.2.8.  CLOSE Command . . . . . . . . . . . . . . . . . . . .  33
       3.2.9.  UID EXPUNGE Command . . . . . . . . . . . . . . . . .  34
       3.2.10. VANISHED Response . . . . . . . . . . . . . . . . . .  35
       3.2.11. CLOSED Response Code  . . . . . . . . . . . . . . . .  38
   4.  Long Command Lines (Update to RFC 2683) . . . . . . . . . . .  39
   5.  QRESYNC Server Implementation Considerations  . . . . . . . .  39
     5.1.  Server Implementations That Don't Store Extra State . . .  39
     5.2.  Server Implementations Storing Minimal State  . . . . . .  40
     5.3.  Additional State Required on the Server . . . . . . . . .  40
   6.  Updated Synchronization Sequence  . . . . . . . . . . . . . .  41
   7.  Formal Syntax . . . . . . . . . . . . . . . . . . . . . . . .  44
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .  48
   9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  48
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .  48
     10.1.  Normative References . . . . . . . . . . . . . . . . . .  48
     10.2.  Informative References . . . . . . . . . . . . . . . . .  49
   Appendix A.  Changes since RFC 4551 . . . . . . . . . . . . . . .  50
   Appendix B.  Changes since RFC 5162 . . . . . . . . . . . . . . .  50
   Appendix C.  Acknowledgements . . . . . . . . . . . . . . . . . .  51






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1.  Introduction

   Often, multiple IMAP [RFC3501] clients need to coordinate changes to
   a common IMAP mailbox.  Examples include different clients working on
   behalf of the same user and clients representing multiple users
   accessing shared mailboxes.  These clients need a mechanism to
   synchronize state changes for messages within the mailbox.  The
   Conditional Store ("CONDSTORE") facility allows a client to quickly
   resynchronize mailbox flag changes.

   The Conditional Store facility also provides a protected update
   mechanism for message state information that can detect and resolve
   conflicts between multiple writing mail clients.  The mechanism can
   be used to guarantee that only one client can change the message
   state at any given time.  For example, this can be used by multiple
   clients that treat a mailbox as a message queue.

   The Conditional Store facility is provided by associating a
   modification sequence (mod-sequence) with every IMAP message.  This
   is updated whenever metadata (such as a message flag) is modified.

   The CONDSTORE extension is described in more detail in Section 3.1.

   The CONDSTORE extension gives a disconnected client the ability to
   quickly resynchronize IMAP flag changes for previously seen messages.
   This can be done using the CHANGEDSINCE FETCH modifier once a mailbox
   is opened.  In order for the client to discover which messages have
   been expunged, the client still has to issue a UID FETCH or a UID
   SEARCH command.  The Quick Mailbox Resynchronization (QRESYNC) IMAP
   extension is an extension to CONDSTORE that allows a reconnecting
   client to perform full resynchronization, including discovery of
   expunged messages, in a single round trip.  QRESYNC also introduces a
   new response, VANISHED, that allows for a more compact representation
   of a list of expunged messages.

   QRESYNC can be useful for mobile clients that can experience frequent
   disconnects caused by environmental factors (such as battery life,
   signal strength, etc.).  Such clients need a way to quickly reconnect
   to the IMAP server, while minimizing delay experienced by the user as
   well as the amount of traffic generated by resynchronization.

   By extending the SELECT command to perform the additional
   resynchronization, this also allows clients to reduce concurrent
   connections to the IMAP server held purely for the sake of avoiding
   the resynchronization.

   The QRESYNC extension is described in more detail in Section 3.2.




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2.  Requirements Notation

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [RFC2119].

   In the examples that follow, "C:" and "S:" indicate lines sent by the
   client and server, respectively.  If a single "C:" or "S:" label
   applies to multiple lines, then the line breaks between those lines
   are for editorial clarity only and are not part of the actual
   protocol exchange.  The five characters [...] means that something
   has been elided.

   Formal syntax is defined using ABNF [RFC5234].

   The term "metadata" or "metadata item" is used throughout this
   document.  It refers to any system- or user-defined keyword.  If the
   server supports the IMAP ANNOTATE-EXPERIMENT-1 extension [RFC5257],
   then metadata also includes message annotations.  Future documents
   may extend "metadata" to include other dynamic message data.

   Some IMAP mailboxes are private, accessible only to the owning user.
   Other mailboxes are not, either because the owner has set an Access
   Control List [RFC4314] that permits access by other users or because
   it is a shared mailbox.  Let's call a metadata item "shared" for the
   mailbox if any changes to the metadata items are persistent and
   visible to all other users accessing the mailbox.  Otherwise, the
   metadata item is called "private".  Note that private metadata items
   are still visible to all sessions accessing the mailbox as the same
   user.  Also, note that different mailboxes may have different
   metadata items as shared.

   See Section 3.1 for the definition of a "CONDSTORE-aware client" and
   a "CONDSTORE enabling command".

   Understanding of the IMAP message sequence numbers and UIDs (see
   Section 2.3.1 of [RFC3501]) and the EXPUNGE response (see
   Section 7.4.1 of [RFC3501]) is essential when reading this document.

3.  IMAP Protocol Changes

3.1.  CONDSTORE Extension

   An IMAP server that supports CONDSTORE MUST associate a positive
   unsigned 63-bit (*) value, called a mod-sequence, with every IMAP
   message.  This is an opaque value updated by the server whenever a
   metadata item is modified.  The server MUST guarantee that each STORE
   command performed on the same mailbox (including simultaneous stores



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   to different metadata items from different connections) will get a
   different mod-sequence value.  Also, for any two successful STORE
   operations performed in the same session on the same mailbox, the
   mod-sequence of the second completed operation MUST be greater than
   the mod-sequence of the first completed operation.  Note that the
   latter rule disallows the direct use of the system clock as a mod-
   sequence because if system time changes (e.g., an NTP [NTP] client
   adjusting the time), the next generated value might be less than the
   previous one.

   (*) Note: RFC 4551 defined mod-sequences as unsigned 64-bit values.
   In order to make implementations on various platforms (such as Java)
   easier, this version of the document redefines them as unsigned
   63-bit values.

   These rules allow a client to list all metadata changes since a well-
   known point in time, as well as to perform conditional metadata
   modifications based on an assumption that the metadata state hasn't
   changed for a particular message.

   In particular, mod-sequences allow a client that supports the
   CONDSTORE extension to determine if a message metadata has changed
   since some known moment.  Whenever the state of a flag changes (i.e.,
   the flag is added where previously it wasn't set, or the flag is
   removed where previously it was set), the value of the modification
   sequence for the message MUST be updated.  Setting a flag that is
   already set, or clearing a flag that is not set, SHOULD NOT change
   the mod-sequence.

   When a message is appended to a mailbox (via the IMAP APPEND command,
   COPY to the mailbox, or using an external mechanism), the server
   generates a new modification sequence that is higher than the highest
   modification sequence of all messages in the mailbox and assigns it
   to the appended message.

   The server MAY store separate (per-message) modification sequence
   values for different metadata items.  If the server does so, per-
   message mod-sequence is the highest mod-sequence of all metadata
   items accessible to the currently logged-in user for the specified
   message.

   The server that supports CONDSTORE is not required to be able to
   store mod-sequences for every available mailbox.  Section 3.1.2.2
   describes how the server may act if a particular mailbox doesn't
   support the persistent storage of mod-sequences.






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   CONDSTORE makes the following changes to the IMAP4 protocol:

   a.  adds the UNCHANGEDSINCE STORE modifier.

   b.  adds the MODIFIED response code that is used with an OK response
       to the STORE command.  (It can also be used in a NO response.)

   c.  adds a new MODSEQ message data item for use with the FETCH
       command.

   d.  adds the CHANGEDSINCE FETCH modifier.

   e.  adds a new MODSEQ search criterion.

   f.  extends the syntax of untagged SEARCH and ESEARCH responses to
       include mod-sequence.

   g.  adds new OK untagged responses (HIGHESTMODSEQ and NOMODSEQ) for
       the SELECT and EXAMINE commands.

   h.  defines an additional CONDSTORE parameter to SELECT/EXAMINE
       commands.

   i.  adds the HIGHESTMODSEQ status data item to the STATUS command.

   A client supporting the CONDSTORE extension indicates its willingness
   to receive mod-sequence updates in all untagged FETCH responses by
   issuing one of the following, which are called "CONDSTORE enabling
   commands":

   o  a SELECT or EXAMINE command with the CONDSTORE parameter,

   o  a STATUS (HIGHESTMODSEQ) command,

   o  a FETCH or SEARCH command that includes the MODSEQ message data
      item,

   o  a FETCH command with the CHANGEDSINCE modifier,

   o  a STORE command with the UNCHANGEDSINCE modifier, or

   o  an ENABLE command containing "CONDSTORE" as one of the parameters.
      (This option only applies when the client is communicating with a
      server that also implements the ENABLE extension [RFC5161].)

   Once a client issues a CONDSTORE enabling command, it has announced
   itself as a "CONDSTORE-aware client".  The server MUST then include
   mod-sequence data in all subsequent untagged FETCH responses (until



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   the connection is closed), whether they were caused by a regular
   STORE, a STORE with an UNCHANGEDSINCE modifier, or an external agent.

   A future extension to this document may extend the list of CONDSTORE
   enabling commands.  A first CONDSTORE enabling command executed in
   the session with a mailbox selected MUST cause the server to return
   HIGHESTMODSEQ (Section 3.1.2.1) for the mailbox (if any is selected),
   unless the server has sent a NOMODSEQ (Section 3.1.2.2) response code
   when the currently selected mailbox was selected.

3.1.1.  Advertising Support for CONDSTORE

   The Conditional STORE extension is present in any IMAP4
   implementation that returns "CONDSTORE" as one of the supported
   capabilities in the CAPABILITY command response.

3.1.2.  New OK Untagged Responses for SELECT and EXAMINE

   This document adds two new response codes: HIGHESTMODSEQ and
   NOMODSEQ.  One of these two response codes MUST be returned in an OK
   untagged response for any successful SELECT/EXAMINE command issued
   after a CONDSTORE enabling command.

   When opening a mailbox, the server must check if the mailbox supports
   the persistent storage of mod-sequences.  If the mailbox supports the
   persistent storage of mod-sequences and the mailbox open operation
   succeeds, the server MUST send an OK untagged response, including the
   HIGHESTMODSEQ response code.  If the persistent storage for the
   mailbox is not supported, the server MUST send an OK untagged
   response, including the NOMODSEQ response code instead.

3.1.2.1.  HIGHESTMODSEQ Response Code

   This document adds a new response code that is returned in an OK
   untagged response for the SELECT and EXAMINE commands.  Once a
   CONDSTORE enabling command is issued, a server supporting the
   persistent storage of mod-sequences for the mailbox MUST send an OK
   untagged response, including the HIGHESTMODSEQ response code with
   every successful SELECT or EXAMINE command:

      OK [HIGHESTMODSEQ <mod-sequence-value>]

      where <mod-sequence-value> is the highest mod-sequence value of
      all messages in the mailbox.  When the server changes UIDVALIDITY
      for a mailbox, it doesn't have to keep the same HIGHESTMODSEQ for
      the mailbox.





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   Note that some existing CONDSTORE servers don't start tracking mod-
   sequences or don't report them until after a CONDSTORE enabling
   command is issued.  Because of that, a client wishing to receive
   HIGHESTMODSEQ/NOMODSEQ information must first send a CONDSTORE
   enabling command, for example, by using SELECT/EXAMINE with the
   CONDSTORE parameter (see Section 3.1.8).

   A disconnected client can use the value of HIGHESTMODSEQ to check if
   it has to refetch metadata from the server.  If the UIDVALIDITY value
   has changed for the selected mailbox, the client MUST delete the
   cached value of HIGHESTMODSEQ.  If UIDVALIDITY for the mailbox is the
   same, and if the HIGHESTMODSEQ value stored in the client's cache is
   less than the value returned by the server, then some metadata items
   on the server have changed since the last synchronization, and the
   client needs to update its cache.  The client MAY use SEARCH MODSEQ
   (Section 3.1.5) to find out exactly which metadata items have
   changed.  Alternatively, the client MAY issue FETCH with the
   CHANGEDSINCE modifier (Section 3.1.4.1) in order to fetch data for
   all messages that have metadata items changed since some known
   modification sequence.

   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: * OK [HIGHESTMODSEQ 715194045007]
   S: A142 OK [READ-WRITE] SELECT completed

                                 Example 1

3.1.2.2.  NOMODSEQ Response Code

   Once a CONDSTORE enabling command is issued, 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.  Note that some
   existing CONDSTORE servers don't return NOMODSEQ until after a
   CONDSTORE enabling command is issued.  Because of that, a client
   wishing to receive HIGHESTMODSEQ/NOMODSEQ information must first send
   a CONDSTORE enabling command, for example, by using SELECT/EXAMINE
   with the CONDSTORE parameter (see Section 3.1.8).






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   A server that returned the NOMODSEQ response code for a mailbox MUST
   reject (with a tagged BAD response) any of the following commands
   while the mailbox remains selected:

   o  a FETCH command with the CHANGEDSINCE modifier,

   o  a FETCH or SEARCH command that includes the MODSEQ message data
      item, or

   o  a STORE command with the UNCHANGEDSINCE modifier.

   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: * OK [NOMODSEQ] Sorry, this mailbox format doesn't support
       modsequences
   S: A142 OK [READ-WRITE] SELECT completed

                                 Example 2

3.1.3.  STORE and UID STORE Commands

   This document defines the following STORE modifier (see Section 2.5
   of [RFC4466]):

   UNCHANGEDSINCE <mod-sequence>

   For each message specified in the message set, the server performs
   the following.  If the mod-sequence of every metadata item of the
   message affected by the STORE/UID STORE is equal to or less than the
   specified UNCHANGEDSINCE value, then the requested operation (as
   described by the message data item) is performed.  If the operation
   is successful, the server MUST update the mod-sequence attribute of
   the message.  An untagged FETCH response MUST be sent, even if the
   .SILENT suffix is specified, and the response MUST include the MODSEQ
   message data item.  This is required to update the client's cache
   with the correct mod-sequence values.  See Section 3.1.4.2 for more
   details.

   However, if the mod-sequence of any metadata item of the message is
   greater than the specified UNCHANGEDSINCE value, then the requested
   operation MUST NOT be performed.  In this case, the mod-sequence




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   attribute of the message is not updated, and the message number (or
   unique identifier in the case of the UID STORE command) is added to
   the list of messages that failed the UNCHANGEDSINCE test.

   When the server finishes performing the operation on all the messages
   in the message set, it checks for a non-empty list of messages that
   failed the UNCHANGEDSINCE test.  If this list is non-empty, the
   server MUST return in the tagged response a MODIFIED response code.
   The MODIFIED response code includes the message set (for STORE) or
   set of UIDs (for UID STORE) of all messages that failed the
   UNCHANGEDSINCE test.

   All messages pass the UNCHANGEDSINCE test.

   C: a103 UID STORE 6,4,8 (UNCHANGEDSINCE 12121230045)
       +FLAGS.SILENT (\Deleted)
   S: * 1 FETCH (UID 4 MODSEQ (12121231000))
   S: * 2 FETCH (UID 6 MODSEQ (12121230852))
   S: * 4 FETCH (UID 8 MODSEQ (12121230956))
   S: a103 OK Conditional Store completed

                                 Example 3

   C: a104 STORE * (UNCHANGEDSINCE 12121230045) +FLAGS.SILENT
       (\Deleted $Processed)
   S: * 50 FETCH (MODSEQ (12111230047))
   S: a104 OK Store (conditional) completed

                                 Example 4

   C: c101 STORE 50 (UNCHANGEDSINCE 12121230045) -FLAGS.SILENT
       (\Deleted)
   S: * OK [HIGHESTMODSEQ 12111230047]
   S: * 50 FETCH (MODSEQ (12111230048))
   S: c101 OK Store (conditional) completed

   The HIGHESTMODSEQ response code was sent by the server presumably
   because this was the first CONDSTORE enabling command.

                                 Example 5

   The failure of the conditional STORE operation for any particular
   message or messages (7 in this example) does not stop the server from
   finding all messages that fail the UNCHANGEDSINCE test.  All such
   messages are returned in the MODIFIED response code.






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   C: d105 STORE 7,5,9 (UNCHANGEDSINCE 320162338)
       +FLAGS.SILENT (\Deleted)
   S: * 5 FETCH (MODSEQ (320162350))
   S: d105 OK [MODIFIED 7,9] Conditional STORE failed

                                 Example 6

   Same as above, but the server follows the SHOULD recommendation in
   Section 6.4.6 of [RFC3501].

   C: d105 STORE 7,5,9 (UNCHANGEDSINCE 320162338)
       +FLAGS.SILENT (\Deleted)
   S: * 7 FETCH (MODSEQ (320162342) FLAGS (\Seen \Deleted))
   S: * 5 FETCH (MODSEQ (320162350))
   S: * 9 FETCH (MODSEQ (320162349) FLAGS (\Answered))
   S: d105 OK [MODIFIED 7,9] Conditional STORE failed

   Use of UNCHANGEDSINCE with a modification sequence of 0 always fails
   if the metadata item exists.  A system flag MUST always be considered
   existent, whether it was set or not.

                                 Example 7

   C: a102 STORE 12 (UNCHANGEDSINCE 0)
       +FLAGS.SILENT ($MDNSent)
   S: a102 OK [MODIFIED 12] Conditional STORE failed

   The client has tested the presence of the $MDNSent user-defined
   keyword.

                                 Example 8

   Note: A client trying to make an atomic change to the state of a
   particular metadata item (or a set of metadata items) MUST be
   prepared to deal with the case when the server returns the MODIFIED
   response code if the state of the metadata item being watched hasn't
   changed (but the state of some other metadata item has).  This is
   necessary because some servers don't store separate mod-sequences for
   different metadata items.  However, a server implementation SHOULD
   avoid generating spurious MODIFIED responses for +FLAGS/-FLAGS STORE
   operations, even when the server stores a single mod-sequence per
   message.  Section 3.1.12 describes how this can be achieved.

   Unless the server has included an unsolicited FETCH to update the
   client's knowledge about messages that have failed the UNCHANGEDSINCE
   test, upon receipt of the MODIFIED response code, the client SHOULD
   try to figure out if the required metadata items have indeed changed




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   by issuing the FETCH or NOOP command.  It is RECOMMENDED that the
   server avoids the need for the client to do that by sending an
   unsolicited FETCH response (see Examples 9 and 10).

   If the required metadata items haven't changed, the client SHOULD
   retry the command with the new mod-sequence.  The client needs to
   allow for a reasonable number of retries (at least 2).

   In the example below, the server returns the MODIFIED response code
   without sending information describing why the STORE UNCHANGEDSINCE
   operation has failed.

   C: a106 STORE 100:150 (UNCHANGEDSINCE 212030000000)
       +FLAGS.SILENT ($Processed)
   S: * 100 FETCH (MODSEQ (303181230852))
   S: * 102 FETCH (MODSEQ (303181230852))
      ...
   S: * 150 FETCH (MODSEQ (303181230852))
   S: a106 OK [MODIFIED 101] Conditional STORE failed

   The flag $Processed was set on the message 101...

   C: a107 NOOP
   S: * 101 FETCH (MODSEQ (303011130956) FLAGS ($Processed))
   S: a107 OK

                                 Example 9

   Or, the flag hasn't changed, but another has (note that this server
   behavior is discouraged.  Server implementers should also see
   Section 3.1.12)...

   C: b107 NOOP
   S: * 101 FETCH (MODSEQ (303011130956) FLAGS (\Deleted \Answered))
   S: b107 OK

   ...and the client retries the operation for the message 101 with
   the updated UNCHANGEDSINCE value.

   C: b108 STORE 101 (UNCHANGEDSINCE 303011130956)
       +FLAGS.SILENT ($Processed)
   S: * 101 FETCH (MODSEQ (303181230852))
   S: b108 OK Conditional Store completed

   Same as above, but the server avoids the need for the client to poll
   for changes.





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   The flag $Processed was set on the message 101 by another
   client...

   C: a106 STORE 100:150 (UNCHANGEDSINCE 212030000000)
       +FLAGS.SILENT ($Processed)
   S: * 100 FETCH (MODSEQ (303181230852))
   S: * 101 FETCH (MODSEQ (303011130956) FLAGS ($Processed))
   S: * 102 FETCH (MODSEQ (303181230852))
   ...
   S: * 150 FETCH (MODSEQ (303181230852))
   S: a106 OK [MODIFIED 101] Conditional STORE failed

                                Example 10

   Or, the flag hasn't changed, but another has (note that this server
   behavior is discouraged.  Server implementers should also see
   Section 3.1.12)...


   C: a106 STORE 100:150 (UNCHANGEDSINCE 212030000000)
       +FLAGS.SILENT ($Processed)
   S: * 100 FETCH (MODSEQ (303181230852))
   S: * 101 FETCH (MODSEQ (303011130956) FLAGS (\Deleted \Answered))
   S: * 102 FETCH (MODSEQ (303181230852))
   ...
   S: * 150 FETCH (MODSEQ (303181230852))
   S: a106 OK [MODIFIED 101] Conditional STORE failed

   ...and the client retries the operation for the message 101 with
   the updated UNCHANGEDSINCE value.

   C: b108 STORE 101 (UNCHANGEDSINCE 303011130956)
       +FLAGS.SILENT ($Processed)
   S: * 101 FETCH (MODSEQ (303181230852))
   S: b108 OK Conditional Store completed

   Or, the flag hasn't changed, but another has (nice server behavior.
   Server implementers should also see Section 3.1.12)...

   C: a106 STORE 100:150 (UNCHANGEDSINCE 212030000000)
       +FLAGS.SILENT ($Processed)
   S: * 100 FETCH (MODSEQ (303181230852))
   S: * 101 FETCH (MODSEQ (303011130956) FLAGS ($Processed \Deleted
       \Answered))
   S: * 102 FETCH (MODSEQ (303181230852))
   ...
   S: * 150 FETCH (MODSEQ (303181230852))
   S: a106 OK Conditional STORE completed



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   The following example is based on the example from Section 4.2.3 of
   [RFC2180] and demonstrates that the MODIFIED response code MAY also
   be returned in the tagged NO response.

   The client tries to conditionally STORE flags on a mixture of
   expunged and non-expunged messages; one message fails the
   UNCHANGEDSINCE test.

   C: B001 STORE 1:7 (UNCHANGEDSINCE 320172338) +FLAGS (\SEEN)
   S: * 1 FETCH (MODSEQ (320172342) FLAGS (\SEEN))
   S: * 3 FETCH (MODSEQ (320172342) FLAGS (\SEEN))
   S: B001 NO [MODIFIED 2] Some of the messages no longer exist.

   C: B002 NOOP
   S: * 4 EXPUNGE
   S: * 4 EXPUNGE
   S: * 4 EXPUNGE
   S: * 4 EXPUNGE
   S: * 2 FETCH (MODSEQ (320172340) FLAGS (\Deleted \Answered))
   S: B002 OK NOOP Completed.

   By receiving FETCH responses for messages 1 and 3, and EXPUNGE
   responses that indicate that messages 4 through 7 have been
   expunged, the client retries the operation only for message 2.
   The updated UNCHANGEDSINCE value is used.

   C: b003 STORE 2 (UNCHANGEDSINCE 320172340) +FLAGS (\Seen)
   S: * 2 FETCH (MODSEQ (320180050) FLAGS (\SEEN \Flagged))
   S: b003 OK Conditional Store completed

                                Example 11

   Note: If a message is specified multiple times in the message set,
   and the server doesn't internally eliminate duplicates from the
   message set, it MUST NOT fail the conditional STORE operation for the
   second (or subsequent) occurrence of the message if the operation
   completed successfully for the first occurrence.  For example, if the
   client specifies:

      e105 STORE 7,3:9 (UNCHANGEDSINCE 12121230045) +FLAGS.SILENT
      (\Deleted)

   the server must not fail the operation for message 7 as part of
   processing "3:9" if it succeeded when message 7 was processed the
   first time.






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   As specified in Section 3.1, once the client specifies the
   UNCHANGEDSINCE modifier in a STORE command, the server starts
   including the MODSEQ FETCH response data items in all subsequent
   unsolicited FETCH responses.

   This document also changes the behavior of the server when it has
   performed a STORE or UID STORE command and the UNCHANGEDSINCE
   modifier is not specified.  If the operation is successful for a
   message, the server MUST update the mod-sequence attribute of the
   message.  The server is REQUIRED to include the mod-sequence value
   whenever it decides to send the unsolicited FETCH response to all
   CONDSTORE-aware clients that have opened the mailbox containing the
   message.

   Server implementers should also see Section 3.1.11 for additional
   quality of implementation issues related to the STORE command.

3.1.4.  FETCH and UID FETCH Commands

3.1.4.1.  CHANGEDSINCE FETCH Modifier

   This document defines the following FETCH modifier (see Section 2.4
   of [RFC4466]):

   CHANGEDSINCE <mod-sequence>:  The CHANGEDSINCE FETCH modifier allows
      the client to further subset the list of messages described by the
      sequence set.  The information described by message data items is
      only returned for messages that have a mod-sequence bigger than
      <mod-sequence>.

      When the CHANGEDSINCE FETCH modifier is specified, it implicitly
      adds the MODSEQ FETCH message data item (Section 3.1.4.2).

   C: s100 UID FETCH 1:* (FLAGS) (CHANGEDSINCE 12345)
   S: * 1 FETCH (UID 4 MODSEQ (65402) FLAGS (\Seen))
   S: * 2 FETCH (UID 6 MODSEQ (75403) FLAGS (\Deleted))
   S: * 4 FETCH (UID 8 MODSEQ (29738) FLAGS ($NoJunk $AutoJunk
       $MDNSent))
   S: s100 OK FETCH completed

                                Example 12










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3.1.4.2.  MODSEQ Message Data Item in FETCH Command

   CONDSTORE adds a MODSEQ message data item to the FETCH command.  The
   MODSEQ message data item allows clients to retrieve mod-sequence
   values for a range of messages in the currently selected mailbox.

   As specified in Section 3.1, once the client has specified the MODSEQ
   message data item in a FETCH request, the server starts including the
   MODSEQ FETCH response data items in all subsequent unsolicited FETCH
   responses.

   Syntax:  MODSEQ

      The MODSEQ message data item causes the server to return MODSEQ
      FETCH response data items.

   Syntax:  MODSEQ ( <permsg-modsequence> )

      MODSEQ response data items contain per-message mod-sequences.

      The MODSEQ response data item is returned if the client issued
      FETCH with the MODSEQ message data item.  It also allows the
      server to notify the client about mod-sequence changes caused by
      conditional STOREs (Section 3.1.3) and/or changes caused by
      external sources.

   C: a FETCH 1:3 (MODSEQ)
   S: * 1 FETCH (MODSEQ (624140003))
   S: * 2 FETCH (MODSEQ (624140007))
   S: * 3 FETCH (MODSEQ (624140005))
   S: a OK Fetch complete

   In this example, the client requests per-message mod-sequences for a
   set of messages.

                                Example 13

   Servers that only support the CONDSTORE extension (and not QRESYNC)
   SHOULD comply with requirements from Section 3.2.4.

   When a flag for a message is modified in a different session, the
   server sends an unsolicited FETCH response containing the mod-
   sequence for the message, as demonstrated in Example 14.  Note that
   when the server also supports the QRESYNC extension (Section 3.2.3)
   and a CONDSTORE enabling command has been issued, all FETCH responses
   in Example 14 must also include UID FETCH items as prescribed by
   Section 3.2.4.




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   (Session 1, authenticated as the user "alex".)  The user adds a
   shared flag \Deleted:

       C: A142 SELECT INBOX
       ...
       S: * FLAGS (\Answered \Flagged \Deleted \Seen \Draft)
       S: * OK [PERMANENTFLAGS (\Answered \Deleted \Seen \*)] Limited
       ...
       C: A160 STORE 7 +FLAGS.SILENT (\Deleted)
       S: * 7 FETCH (MODSEQ (2121231000))
       S: A160 OK Store completed

   (Session 2, also authenticated as the user "alex".)  Any changes
   to flags are always reported to all sessions authenticated as the
   same user as in session 1.

       C: C180 NOOP
       S: * 7 FETCH (FLAGS (\Deleted \Answered) MODSEQ (12121231000))
       S: C180 OK Noop completed

   (Session 3, authenticated as the user "andrew".)  As \Deleted is a
   shared flag, changes in session 1 are also reported in session 3:

       C: D210 NOOP
       S: * 7 FETCH (FLAGS (\Deleted \Answered) MODSEQ (12121231000))
       S: D210 OK Noop completed

   The user modifies a private flag, \Seen, in session 1...

       C: A240 STORE 7 +FLAGS.SILENT (\Seen)
       S: * 7 FETCH (MODSEQ (12121231777))
       S: A240 OK Store completed

   ...which is only reported in session 2...

       C: C270 NOOP
       S: * 7 FETCH (FLAGS (\Deleted \Answered \Seen) MODSEQ
           (12121231777))
       S: C270 OK Noop completed

   ...but not in session 3.

       C: D300 NOOP
       S: D300 OK Noop completed







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   And, finally, the user removes flags \Answered (shared) and \Seen
   (private) in session 1.

       C: A330 STORE 7 -FLAGS.SILENT (\Answered \Seen)
       S: * 7 FETCH (MODSEQ (12121245160))
       S: A330 OK Store completed

   Both changes are reported in session 2...

       C: C360 NOOP
       S: * 7 FETCH (FLAGS (\Deleted) MODSEQ (12121245160))
       S: C360 OK Noop completed

   ...and only changes to shared flags are reported in session 3.

       C: D390 NOOP
       S: * 7 FETCH (FLAGS (\Deleted) MODSEQ (12121245160))
       S: D390 OK Noop completed

                                Example 14

   Server implementers should also see Section 3.1.11 for additional
   quality of implementation issues related to the FETCH command.

3.1.5.  MODSEQ Search Criterion in SEARCH

   The MODSEQ criterion for the SEARCH (or UID SEARCH) command allows a
   client to search for the metadata items that were modified since a
   specified moment.

   Syntax: MODSEQ [<entry-name> <entry-type-req>] <mod-sequence-valzer>

      Messages that have modification values that are equal to or
      greater than <mod-sequence-valzer>.  This allows a client, for
      example, to find out which messages contain metadata items that
      have changed since the last time it updated its disconnected
      cache.  The client may also specify <entry-name> (name of the
      metadata item) and <entry-type-req> (type of metadata item) before
      <mod-sequence-valzer>.  <entry-type-req> can be one of "shared",
      "priv" (private), or "all".  The last means that the server MUST
      use the biggest value among "priv" and "shared" mod-sequences for
      the metadata item.  If the server doesn't store separate mod-
      sequences for different metadata items, it MUST ignore <entry-
      name> and <entry-type-req>.  Otherwise, the server should use them
      to narrow down the search.






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      For a flag <flagname>, the corresponding <entry-name> has the form
      "/flags/<flagname>".  Note that the leading "\" character that
      denotes a system flag has to be escaped as per Section 4.3 of
      [RFC3501], as <entry-name> uses the syntax for quoted strings (see
      the examples below).

   If the client specifies a MODSEQ criterion in a SEARCH (or UID
   SEARCH) command and the server returns a non-empty SEARCH result, the
   server MUST also append (to the end of the untagged SEARCH response)
   the highest mod-sequence for all messages being returned.  See also
   Section 3.1.6.  Note that other IMAP extensions such as ESEARCH
   [RFC4731] can override this requirement (see Section 3.1.10 for more
   details.)

   C: a SEARCH MODSEQ "/flags/\\draft" all 620162338
   S: * SEARCH 2 5 6 7 11 12 18 19 20 23 (MODSEQ 917162500)
   S: a OK Search complete

   In the above example, the message numbers of any messages having a
   mod-sequence equal to or greater than 620162338 for the "\Draft" flag
   are returned in the search results.

                                Example 15

   C: t SEARCH OR NOT MODSEQ 720162338 LARGER 50000
   S: * SEARCH
   S: t OK Search complete, nothing found

                                Example 16

3.1.6.  Modified SEARCH Untagged Response

   Data:       zero or more numbers
               mod-sequence value (omitted if no match)

   This document extends the syntax of the untagged SEARCH response to
   include the highest mod-sequence for all messages being returned.

   If a client specifies a MODSEQ criterion in a SEARCH (or UID SEARCH)
   command and the server returns a non-empty SEARCH result, the server
   MUST also append (to the end of the untagged SEARCH response) the
   highest mod-sequence for all messages being returned.  See
   Section 3.1.5 for examples.








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3.1.7.  HIGHESTMODSEQ Status Data Items

   This document defines a new status data item:

   HIGHESTMODSEQ:  The highest mod-sequence value of all messages in the
      mailbox.  This is the same value that is returned by the server in
      the HIGHESTMODSEQ response code in an OK untagged response (see
      Section 3.1.2.1).  If the server doesn't support the persistent
      storage of mod-sequences for the mailbox (see Section 3.1.2.2),
      the server MUST return 0 as the value of the HIGHESTMODSEQ status
      data item.

   C: A042 STATUS blurdybloop (UIDNEXT MESSAGES HIGHESTMODSEQ)
   S: * STATUS blurdybloop (MESSAGES 231 UIDNEXT 44292
       HIGHESTMODSEQ 7011231777)
   S: A042 OK STATUS completed

                                Example 17

3.1.8.  CONDSTORE Parameter to SELECT and EXAMINE

   The CONDSTORE extension defines a single optional select parameter,
   "CONDSTORE", which tells the server that it MUST include the MODSEQ
   FETCH response data items in all subsequent unsolicited FETCH
   responses.

   The CONDSTORE parameter to SELECT/EXAMINE helps avoid a race
   condition that might arise when one or more metadata items are
   modified in another session after the server has sent the
   HIGHESTMODSEQ response code and before the client was able to issue a
   CONDSTORE enabling command.

   C: A142 SELECT INBOX (CONDSTORE)
   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: * OK [HIGHESTMODSEQ 715194045007]
   S: A142 OK [READ-WRITE] SELECT completed, CONDSTORE is now enabled

                                Example 18







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3.1.9.  Interaction with IMAP SORT and THREAD Extensions

   The MODSEQ Search Criterion (see Section 3.1.5) causes modifications
   to SORT [RFC5256] responses similar to modifications to SEARCH
   responses defined in Section 3.1.6:

   SORT Response Data:       zero or more numbers
                             mod-sequence value (omitted if no match)

   This document extends the syntax of the untagged SORT response to
   include the highest mod-sequence for all messages being returned.

   If a client specifies a MODSEQ criterion in a SORT (or UID SORT)
   command and the server returns a non-empty SORT result, the server
   MUST also append (to the end of the untagged SORT response) the
   highest mod-sequence for all messages being returned.  Note that
   other IMAP extensions such as ESORT [RFC5267] can override this
   requirement (see Section 3.1.10 for more details.)

   THREAD commands that include a MODSEQ Search Criterion return THREAD
   responses as specified in [RFC5256], i.e., THREAD responses are
   unchanged by the CONDSTORE extension.

3.1.10.  Interaction with IMAP ESORT and ESEARCH Extensions

   If a client specifies a MODSEQ criterion in an extended SEARCH (or
   extended UID SEARCH) [RFC4731] command and the server returns a non-
   empty SEARCH result, the server MUST return the ESEARCH response
   containing the MODSEQ result option as defined in Section 3.2 of
   [RFC4731].

   C: a SEARCH RETURN (ALL) MODSEQ 1234
   S: * ESEARCH (TAG "a") ALL 1:3,5 MODSEQ 1236
   S: a OK Extended SEARCH completed

                                Example 19

   If a client specifies a MODSEQ criterion in an extended SORT (or
   extended UID SORT) [RFC5267] command and the server returns a non-
   empty SORT result, the server MUST return the ESEARCH response
   containing the MODSEQ result option defined in Section 3.2 of
   [RFC4731].

   C: a SORT RETURN (ALL) (DATE) UTF-8 MODSEQ 1234
   S: * ESEARCH (TAG "a") ALL 5,3,2,1 MODSEQ 1236
   S: a OK Extended SORT completed

                                Example 20



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3.1.11.  Additional Quality-of-Implementation Issues

   Server implementations should follow the following rule, which
   applies to any successfully completed STORE/UID STORE (with and
   without an UNCHANGEDSINCE modifier), as well as to a FETCH command
   that implicitly sets the \Seen flag:

      Adding the flag when it is already present or removing it when it
      is not present SHOULD NOT change the mod-sequence.

   This will prevent spurious client synchronization requests.

   However, note that client implementers MUST NOT rely on this server
   behavior.  A client can't distinguish between the case when a server
   has violated the SHOULD mentioned above and when one or more clients
   set and unset (or unset and set) the flag in another session.

3.1.12.  CONDSTORE Server Implementation Considerations

   This section describes how a server implementation that doesn't store
   separate per-metadata mod-sequences for different metadata items can
   avoid sending the MODIFIED response to any of the following
   conditional STORE operations:

      +FLAGS

      -FLAGS

      +FLAGS.SILENT

      -FLAGS.SILENT

   Note that the optimization described in this section can't be
   performed in case of a conditional STORE FLAGS (without "+" or "-")
   operation.

   Let's use the following example.  The client has issued:

   C: a106 STORE 100:150 (UNCHANGEDSINCE 212030000000)
       +FLAGS.SILENT ($Processed)

   When the server receives the command and parses it successfully, it
   iterates through the message set and tries to execute the conditional
   STORE command for each message.







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   Each server internally works as a client, i.e., it has to cache the
   current state of all IMAP flags as it is known to the client.  In
   order to report flag changes to the client, the server compares the
   cached values with the values in its database for IMAP flags.

   Imagine that another client has changed the state of a flag \Deleted
   on the message 101 and that the change updated the mod-sequence for
   the message.  The server knows that the mod-sequence for the mailbox
   has changed; however, it also knows that:

   a.  the client is not interested in the \Deleted flag, as it hasn't
       included it in the +FLAGS.SILENT operation and

   b.  the state of the flag $Processed hasn't changed (the server can
       determine this by comparing the cached flag state with the state
       of the flag in the database).

   Therefore, the server doesn't have to report MODIFIED to the client.
   Instead, the server may set the $Processed flag, update the mod-
   sequence for the message 101 once again, and send an untagged FETCH
   response with a new mod-sequence and flags:

   S: * 101 FETCH (MODSEQ (303011130956) FLAGS ($Processed \Deleted
       \Answered))

   See also Section 3.1.11 for additional quality-of-implementation
   issues.

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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RFC 7162                IMAP CONDSTORE & QRESYNC                May 2014


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
































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