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rfc:rfc2180

Network Working Group M. Gahrns Request for Comments: 2180 Microsoft Category: Informational July 1997

               IMAP4 Multi-Accessed Mailbox Practice

Status of this Memo

 This memo provides information for the Internet community.  This memo
 does not specify an Internet standard of any kind.  Distribution of
 this memo is unlimited.

1. Abstract

 IMAP4[RFC-2060] is rich client/server protocol that allows a client
 to access and manipulate electronic mail messages on a server.
 Within the protocol framework, it is possible to have differing
 results for particular client/server interactions. If a protocol does
 not allow for this, it is often unduly restrictive.
 For example, when multiple clients are accessing a mailbox and one
 attempts to delete the mailbox, an IMAP4 server may choose to
 implement a solution based upon server architectural constraints or
 individual preference.
 With this flexibility comes greater client responsibility.  It is not
 sufficient for a client to be written based upon the behavior of a
 particular IMAP server.  Rather the client must be based upon the
 behavior allowed by the protocol.
 By documenting common IMAP4 server practice for the case of
 simultaneous client access to a mailbox, we hope to ensure the widest
 amount of inter-operation between IMAP4 clients and servers.
 The behavior described in this document reflects the practice of some
 existing servers or behavior that the consensus of the IMAP mailing
 list has deemed to be reasonable.  The behavior described within this
 document is believed to be [RFC-2060] compliant. However, this
 document is not meant to define IMAP4 compliance, nor is it an
 exhaustive list of valid IMAP4 behavior. [RFC-2060] must always be
 consulted to determine IMAP4 compliance, especially for server
 behavior not described within this document.

Gahrns Informational [Page 1] RFC 2180 IMAP4 Multi-Accessed Mailbox Practice July 1997

2. Conventions used in this document

 In examples,"C1:", "C2:" and "C3:" indicate lines sent by 3 different
 clients (client #1, client #2 and client #3) that are connected to a
 server.  "S1:", "S2:" and "S3:" indicated lines sent by the server to
 client #1, client #2 and client #3 respectively.
 A shared mailbox, is a mailbox that can be used by multiple users.
 A multi-accessed mailbox, is a mailbox that has multiple clients
 simultaneously accessing it.
 A client is said to have accessed a mailbox after a successful SELECT
 or EXAMINE command.
 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 [RFC-2119].

3. Deletion/Renaming of a multi-accessed mailbox

 If an external agent or multiple clients are accessing a mailbox,
 care must be taken when handling the deletion or renaming of the
 mailbox. Following are some strategies an IMAP server may choose to
 use when dealing with this situation.

3.1. The server MAY fail the DELETE/RENAME command of a multi-accessed

   mailbox
 In some cases, this behavior may not be practical.  For example, if a
 large number of clients are accessing a shared mailbox, the window in
 which no clients have the mailbox accessed may be small or non-
 existent, effectively rendering the mailbox undeletable or
 unrenamable.
 Example:
 <Client #1 and Client #2 have mailbox FOO accessed. Client #1 tries
 to DELETE the mailbox and is refused>
           C1: A001 DELETE FOO
           S1: A001 NO Mailbox FOO is in use by another user.

Gahrns Informational [Page 2] RFC 2180 IMAP4 Multi-Accessed Mailbox Practice July 1997

3.2. The server MAY allow the DELETE command of a multi-accessed

   mailbox, but keep the information in the mailbox available for
   those clients that currently have access to the mailbox.
 When all clients have finished accessing the mailbox, it is
 permanently removed.  For clients that do not already have access to
 the mailbox, the 'ghosted' mailbox would not be available.  For
 example, it would not be returned to these clients in a subsequent
 LIST or LSUB command and would not be a valid mailbox argument to any
 other IMAP command until the reference count of clients accessing the
 mailbox reached 0.
 In some cases, this behavior may not be desirable. For example if
 someone created a mailbox with offensive or sensitive information,
 one might prefer to have the mailbox deleted and all access to the
 information contained within removed immediately, rather than
 continuing to allow access until the client closes the mailbox.
 Furthermore, this behavior, may prevent 'recycling' of the same
 mailbox name until all clients have finished accessing the original
 mailbox.
 Example:
 <Client #1 and Client #2 have mailbox FOO selected. Client #1 DELETEs
 mailbox FOO>
           C1: A001 DELETE FOO
           S1: A001 OK Mailbox FOO is deleted.
 <Client #2 is still able to operate on the deleted mailbox>
           C2: B001 STORE 1 +FLAGS (\Seen)
           S2: * 1 FETCH FLAGS (\Seen)
           S2: B001 OK STORE completed
 <Client #3 which did not have access to the mailbox prior to the
 deletion by client #1 does not have access to the mailbox>
           C3: C001 STATUS FOO (MESSAGES)
           S3: C001 NO Mailbox does not exist
 <Nor is client #3 able to create a mailbox with the name FOO, while
 the reference count is non zero>
           C3: C002 CREATE FOO
           S3: C002 NO Mailbox FOO is still in use. Try again later.

Gahrns Informational [Page 3] RFC 2180 IMAP4 Multi-Accessed Mailbox Practice July 1997

 <Client #2 closes its access to the mailbox, no other clients have
 access to the mailbox FOO and reference count becomes 0>
           C2: B002 CLOSE
           S2: B002 OK CLOSE Completed
 <Now that the reference count on FOO has reached 0, the mailbox name
 can be recycled>
           C3: C003 CREATE FOO
           S3: C003 OK CREATE Completed

3.3. The server MAY allow the DELETE/RENAME of a multi-accessed

   mailbox, but disconnect all other clients who have the mailbox
   accessed by sending a untagged BYE response.
 A server may often choose to disconnect clients in the DELETE case,
 but may choose to implement a "friendlier" method for the RENAME
 case.
 Example:
 <Client #1 and Client #2 have mailbox FOO accessed. Client #1 DELETEs
 the mailbox FOO>
           C1: A002 DELETE FOO
           S1: A002 OK DELETE completed.
 <Server disconnects all other users of the mailbox>
           S2: * BYE Mailbox FOO has been deleted.

3.4. The server MAY allow the RENAME of a multi-accessed mailbox by

   simply changing the name attribute on the mailbox.
 Other clients that have access to the mailbox can continue issuing
 commands such as FETCH that do not reference the mailbox name.
 Clients would discover the renaming the next time they referred to
 the old mailbox name.  Some servers MAY choose to include the
 [NEWNAME] response code in their tagged NO response to a command that
 contained the old mailbox name, as a hint to the client that the
 operation can succeed if the command is issued with the new mailbox
 name.

Gahrns Informational [Page 4] RFC 2180 IMAP4 Multi-Accessed Mailbox Practice July 1997

 Example:
 <Client #1 and Client #2 have mailbox FOO accessed. Client #1 RENAMEs
 the mailbox.>
           C1: A001 RENAME FOO BAR
           S1: A001 OK RENAME completed.
 <Client #2 is still able to do operations that do not reference the
 mailbox name>
           C2: B001 FETCH 2:4 (FLAGS)
           S2: * 2 FETCH . . .
           S2: * 3 FETCH . . .
           S2: * 4 FETCH . . .
           S2: B001 OK FETCH completed
 <Client #2 is not able to do operations that reference the mailbox
 name>
           C2: B002 APPEND FOO {300} C2: Date: Mon, 7 Feb 1994
           21:52:25 0800 (PST) C2: . . .  S2: B002 NO [NEWNAME FOO
           BAR] Mailbox has been renamed

4. Expunging of messages on a multi-accessed mailbox

 If an external agent or multiple clients are accessing a mailbox,
 care must be taken when handling the EXPUNGE of messages.  Other
 clients accessing the mailbox may be in the midst of issuing a
 command that depends upon message sequence numbers.  Because an
 EXPUNGE response can not be sent while responding to a FETCH, STORE
 or SEARCH command, it is not possible to immediately notify the
 client of the EXPUNGE.  This can result in ambiguity if the client
 issues a FETCH, STORE or SEARCH operation on a message that has been
 EXPUNGED.

4.1. Fetching of expunged messages

 Following are some strategies an IMAP server may choose to use when
 dealing with a FETCH command on expunged messages.

Gahrns Informational [Page 5] RFC 2180 IMAP4 Multi-Accessed Mailbox Practice July 1997

 Consider the following scenario:
  1. Client #1 and Client #2 have mailbox FOO selected.
  2. There are 7 messages in the mailbox.
  3. Messages 4:7 are marked for deletion.
  4. Client #1 issues an EXPUNGE, to expunge messages 4:7

4.1.1. The server MAY allow the EXPUNGE of a multi-accessed mailbox but

     keep the messages available to satisfy subsequent FETCH commands
     until it is able to send an EXPUNGE response to each client.
 In some cases, the behavior of keeping "ghosted" messages may not be
 desirable.  For example if a message contained offensive or sensitive
 information, one might prefer to instantaneously remove all access to
 the information, regardless of whether another client is in the midst
 of accessing it.
 Example:  (Building upon the scenario outlined in 4.1.)
 <Client #2 is still able to access the expunged messages because the
 server has kept a 'ghosted' copy of the messages until it is able to
 notify client #2 of the EXPUNGE>
           C2: B001 FETCH 4:7 RFC822
           S2: * 4 FETCH RFC822 . . . (RFC822 info returned)
           S2: * 5 FETCH RFC822 . . . (RFC822 info returned)
           S2: * 6 FETCH RFC822 . . . (RFC822 info returned)
           S2: * 7 FETCH RFC822 . . . (RFC822 info returned)
           S2: B001 OK FETCH Completed
 <Client #2 issues a command where it can get notified of the EXPUNGE>
           C2: B002 NOOP
           S2: * 4 EXPUNGE
           S2: * 4 EXPUNGE
           S2: * 4 EXPUNGE
           S2: * 4 EXPUNGE
           S2: * 3 EXISTS
           S2: B002 OK NOOP Complete
 <Client #2 no longer has access to the expunged messages>
           C2: B003 FETCH 4:7 RFC822
           S2: B003 NO Messages 4:7 are no longer available.

Gahrns Informational [Page 6] RFC 2180 IMAP4 Multi-Accessed Mailbox Practice July 1997

4.1.2 The server MAY allow the EXPUNGE of a multi-accessed mailbox,

    and on subsequent FETCH commands return FETCH responses only for
    non-expunged messages and a tagged NO.
 After receiving a tagged NO FETCH response, the client SHOULD issue a
 NOOP command so that it will be informed of any pending EXPUNGE
 responses.  The client may then either reissue the failed FETCH
 command, or by examining the EXPUNGE response from the NOOP and the
 FETCH response from the FETCH, determine that the FETCH failed
 because of pending expunges.
 Example:  (Building upon the scenario outlined in 4.1.)
 <Client #2 attempts to FETCH a mix of expunged and non-expunged
 messages.  A FETCH response is returned only for then non-expunged
 messages along with a tagged NO>
           C2: B001 FETCH 3:5 ENVELOPE
           S2: * 3 FETCH ENVELOPE . . . (ENVELOPE info returned)
           S2: B001 NO Some of the requested messages no longer exist
 <Upon receiving a tagged NO FETCH response, Client #2 issues a NOOP
 to be informed of any pending EXPUNGE responses>
           C2: B002 NOOP
           S2: * 4 EXPUNGE
           S2: * 4 EXPUNGE
           S2: * 4 EXPUNGE
           S2: * 4 EXPUNGE
           S2: * 3 EXISTS
           S2: B002 OK NOOP Completed.
 <By receiving a FETCH response for message 3, and an EXPUNGE response
 that indicates messages 4:7 have been expunged, the client does not
 need to re-issue the FETCH>

Gahrns Informational [Page 7] RFC 2180 IMAP4 Multi-Accessed Mailbox Practice July 1997

4.1.3 The server MAY allow the EXPUNGE of a multi-accessed mailbox, and

    on subsequent FETCH commands return the usual FETCH responses for
    non-expunged messages, "NIL FETCH Responses" for expunged
    messages, and a tagged OK response.
 If all of the messages in the subsequent FETCH command have been
 expunged, the server SHOULD return only a tagged NO.  In this case,
 the client SHOULD issue a NOOP command so that it will be informed of
 any pending EXPUNGE responses.  The client may then either reissue
 the failed FETCH command, or by examining the EXPUNGE response from
 the NOOP, determine that the FETCH failed because of pending
 expunges.
 "NIL FETCH responses" are a representation of empty data as
 appropriate for the FETCH argument specified.
 Example:
  • 1 FETCH (ENVELOPE (NIL NIL NIL NIL NIL NIL NIL NIL NIL NIL))
  • 1 FETCH (FLAGS ())
  • 1 FETCH (INTERNALDATE "00-Jan-0000 00:00:00 +0000")
  • 1 FETCH (RFC822 "")
  • 1 FETCH (RFC822.HEADER "")
  • 1 FETCH (RFC822.TEXT "")
  • 1 FETCH (RFC822.SIZE 0)
  • 1 FETCH (BODY ("TEXT" "PLAIN" NIL NIL NIL "7BIT" 0 0)
  • 1 FETCH (BODYSTRUCTURE ("TEXT" "PLAIN" NIL NIL NIL "7BIT" 0 0)
  • 1 FETCH (BODY[<section>] "")
  • 1 FETCH (BODY[<section>]<partial> "")
 In some cases, a client may not be able to distinguish between "NIL
 FETCH responses" received because a message was expunged and those
 received because the data actually was NIL.  For example, a  * 5
 FETCH (FLAGS ()) response could be received if no flags were set on
 message 5, or because message 5 was expunged. In a case of potential
 ambiguity, the client SHOULD issue a command such as NOOP to force
 the sending of the EXPUNGE responses to resolve any ambiguity.
 Example:  (Building upon the scenario outlined in 4.1.)
 <Client #2 attempts to access a mix of expunged and non-expunged
 messages.  Normal data is returned for non-expunged message, "NIL
 FETCH responses" are returned for expunged messages>

Gahrns Informational [Page 8] RFC 2180 IMAP4 Multi-Accessed Mailbox Practice July 1997

           C2: B002 FETCH 3:5 ENVELOPE
           S2: * 3 FETCH ENVELOPE . . . (ENVELOPE info returned)
           S2: * 4 FETCH ENVELOPE (NIL NIL NIL NIL NIL NIL NIL NIL
                 NIL NIL)
           S2: * 5 FETCH ENVELOPE (NIL NIL NIL NIL NIL NIL NIL NIL
                 NIL NIL)
           S2: B002 OK FETCH Completed
 <Client #2 attempts to FETCH only expunged messages and receives a
 tagged NO response>
           C2: B002 FETCH 4:7 ENVELOPE
           S2: B002 NO Messages 4:7 have been expunged.

4.1.4 To avoid the situation altogether, the server MAY fail the

    EXPUNGE of a multi-accessed mailbox
 In some cases, this behavior may not be practical.  For example, if a
 large number of clients are accessing a shared mailbox, the window in
 which no clients have the mailbox accessed may be small or non-
 existent, effectively rendering the message unexpungeable.

4.2. Storing of expunged messages

 Following are some strategies an IMAP server may choose to use when
 dealing with a STORE command on expunged messages.

4.2.1 If the ".SILENT" suffix is used, and the STORE completed

    successfully for all the non-expunged messages, the server SHOULD
    return a tagged OK.
 Example:  (Building upon the scenario outlined in 4.1.)
 <Client #2 tries to silently STORE flags on expunged and non-
 expunged messages.  The server sets the flags on the non-expunged
 messages and returns OK>
           C2: B001 STORE 1:7 +FLAGS.SILENT (\SEEN)
           S2: B001 OK

Gahrns Informational [Page 9] RFC 2180 IMAP4 Multi-Accessed Mailbox Practice July 1997

4.2.2. If the ".SILENT" suffix is not used, and only expunged messages

     are referenced, the server SHOULD return only a tagged NO.
 Example:  (Building upon the scenario outlined in 4.1.)
 <Client #2 tries to STORE flags only on expunged messages>
           C2: B001 STORE 5:7 +FLAGS (\SEEN)
           S2: B001 NO  Messages have been expunged

4.2.3. If the ".SILENT" suffix is not used, and a mixture of expunged

     and non-expunged messages are referenced, the server MAY set the
     flags and return a FETCH response for the non-expunged messages
     along with a tagged NO.
 After receiving a tagged NO STORE response, the client SHOULD issue a
 NOOP command so that it will be informed of any pending EXPUNGE
 responses.  The client may then either reissue the failed STORE
 command, or by examining the EXPUNGE responses from the NOOP and
 FETCH responses from the STORE, determine that the STORE failed
 because of pending expunges.
 Example:  (Building upon the scenario outlined in 4.1.)
 <Client #2 tries to STORE flags on a mixture of expunged and non-
 expunged messages>
           C2: B001 STORE 1:7 +FLAGS (\SEEN)
           S2: * FETCH 1 FLAGS (\SEEN)
           S2: * FETCH 2 FLAGS (\SEEN)
           S2: * FETCH 3 FLAGS (\SEEN)
           S2: B001 NO Some of the messages no longer exist.
           C2: B002 NOOP
           S2: * 4 EXPUNGE
           S2: * 4 EXPUNGE
           S2: * 4 EXPUNGE
           S2: * 4 EXPUNGE
           S2: * 3 EXISTS
           S2: B002 OK NOOP Completed.
 <By receiving FETCH responses for messages 1:3, and an EXPUNGE
 response that indicates messages 4:7 have been expunged, the client
 does not need to re-issue the STORE>

Gahrns Informational [Page 10] RFC 2180 IMAP4 Multi-Accessed Mailbox Practice July 1997

4.2.4. If the ".SILENT" suffix is not used, and a mixture of expunged

     and non-expunged messages are referenced, the server MAY return
     an untagged NO and not set any flags.
 After receiving a tagged NO STORE response, the client SHOULD issue a
 NOOP command so that it will be informed of any pending EXPUNGE
 responses.  The client would then re-issue the STORE command after
 updating its message list per any EXPUNGE response.
 If a large number of clients are accessing a shared mailbox, the
 window in which there are no pending expunges may be small or non-
 existent, effectively disallowing a client from setting the flags on
 all messages at once.
 Example:  (Building upon the scenario outlined in 4.1.)
 <Client #2 tries to STORE flags on a mixture of expunged and non-
 expunged messages>
           C2: B001 STORE 1:7 +FLAGS (\SEEN)
           S2: B001 NO  Some of the messages no longer exist.
 <Client #2 issues a NOOP to be informed of the EXPUNGED messages>
           C2: B002 NOOP
           S2: * 4 EXPUNGE
           S2: * 4 EXPUNGE
           S2: * 4 EXPUNGE
           S2: * 4 EXPUNGE
           S2: * 3 EXISTS
           S2: B002 OK NOOP Completed.
 <Client #2 updates its message list and re-issues the STORE on only
 those messages that have not been expunged>
           C2: B003 STORE 1:3 +FLAGS (\SEEN) S2: * FETCH 1 FLAGS
           (\SEEN) S2: * FETCH 2 FLAGS (\SEEN) S2: * FETCH 3 FLAGS
           (\SEEN) S2: B003 OK  STORE Completed

4.3. Searching of expunged messages

 A server MAY simply not return a search response for messages that
 have been expunged and it has not been able to inform the client
 about.  If a client was expecting a particular message to be returned
 in a search result, and it was not, the client SHOULD issue a NOOP
 command to see if the message was expunged by another client.

Gahrns Informational [Page 11] RFC 2180 IMAP4 Multi-Accessed Mailbox Practice July 1997

4.4 Copying of expunged messages

 COPY is the only IMAP4 sequence number command that is safe to allow
 an EXPUNGE response on.  This is because a client is not permitted to
 cascade several COPY commands together. A client is required to wait
 and confirm that the copy worked before issuing another one.

4.4.1 The server MAY disallow the COPY of messages in a multi-access

    mailbox that contains expunged messages.
 Pending EXPUNGE response(s) MUST be returned to the COPY command.
 Example:
           C: A001 COPY 2,4,6,8 FRED
           S: * 4 EXPUNGE
           S: A001 NO COPY rejected, because some of the requested
              messages were expunged
 Note: Non of the above messages are copied because if a COPY command
 is unsuccessful, the server MUST restore the destination mailbox to
 its state before the COPY attempt.

4.4.2 The server MAY allow the COPY of messages in a multi-access

    mailbox that contains expunged messages.
 Pending EXPUNGE response(s) MUST be returned to the COPY command.
 Messages that are copied are messages corresponding to sequence
 numbers before any EXPUNGE response.
 Example:
           C: A001 COPY 2,4,6,8 FRED
           S: * 3 EXPUNGE
           S: A001 OK COPY completed
 In the above example, the messages that are copied to FRED are
 messages 2,4,6,8 at the start of the COPY command.  These are
 equivalent to messages 2,3,5,7 at the end of the COPY command.  The
 EXPUNGE response can't take place until after the messages from the
 COPY command are identified (because of the "no expunge while no
 commands in progress" rule).

Gahrns Informational [Page 12] RFC 2180 IMAP4 Multi-Accessed Mailbox Practice July 1997

 Example:
           C: A001 COPY 2,4,6,8 FRED
           S: * 4 EXPUNGE
           S: A001 OK COPY completed
 In the above example, message 4 was copied before it was expunged,
 and MUST appear in the destination mailbox FRED.

5. Security Considerations

 This document describes behavior of servers that use the IMAP4
 protocol, and as such, has the same security considerations as
 described in [RFC-2060].
 In particular, some described server behavior does not allow for the
 immediate deletion of information when a mailbox is accessed by
 multiple clients.  This may be a consideration when dealing with
 sensitive information where immediate deletion would be preferred.

6. References

 [RFC-2060], Crispin, M., "Internet Message Access Protocol - Version
 4rev1", RFC 2060, University of Washington, December 1996.
 [RFC-2119], Bradner, S., "Key words for use in RFCs to Indicate
 Requirement Levels", RFC 2119, Harvard University, March 1997.

7. Acknowledgments

 This document is the result of discussions on the IMAP4 mailing list
 and is meant to reflect consensus of this group.  In particular,
 Raymond Cheng, Mark Crispin, Jim Evans, Erik Forsberg, Steve Hole,
 Mark Keasling, Barry Leiba, Syd Logan, John Mani, Pat Moran, Larry
 Osterman, Chris Newman, Bart Schaefer, Vladimir Vulovic, and Jack De
 Winter were active participants in this discussion or made
 suggestions to this document.

Gahrns Informational [Page 13] RFC 2180 IMAP4 Multi-Accessed Mailbox Practice July 1997

8. Author's Address

 Mike Gahrns
 Microsoft
 One Microsoft Way
 Redmond, WA, 98072
 Phone: (206) 936-9833
 EMail: mikega@microsoft.com

Gahrns Informational [Page 14]

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