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rfc:rfc8579

Internet Engineering Task Force (IETF) S. Bosch Request for Comments: 8579 Open Xchange Oy Category: Standards Track May 2019 ISSN: 2070-1721

     Sieve Email Filtering: Delivering to Special-Use Mailboxes

Abstract

 The SPECIAL-USE capability of the IMAP protocol (RFC 6154) allows
 clients to identify special-use mailboxes, e.g., where draft or sent
 messages should be put.  This simplifies client configuration.  In
 contrast, the Sieve mail filtering language (RFC 5228) currently has
 no such capability.  This memo defines a Sieve extension that fills
 this gap: it adds a test for checking whether a special-use attribute
 is assigned for a particular mailbox or any mailbox, and it adds the
 ability to file messages into a mailbox identified solely by a
 special-use attribute.

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 7841.
 Information about the current status of this document, any errata,
 and how to provide feedback on it may be obtained at
 https://www.rfc-editor.org/info/rfc8579.

Copyright Notice

 Copyright (c) 2019 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
 (https://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.

Bosch Standards Track [Page 1] RFC 8579 Sieve: Special-Use Mailboxes May 2019

Table of Contents

 1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
 2.  Conventions Used in This Document . . . . . . . . . . . . . .   3
 3.  Test "specialuse_exists"  . . . . . . . . . . . . . . . . . .   3
   3.1.  Equivalent IMAP Operations  . . . . . . . . . . . . . . .   4
 4.  ":specialuse" Argument to "fileinto" Command  . . . . . . . .   5
   4.1.  Mailboxes Created Implicitly by the "fileinto" Command  .   6
   4.2.  Equivalent IMAP Operations  . . . . . . . . . . . . . . .   7
 5.  Sieve Capability Strings  . . . . . . . . . . . . . . . . . .   8
 6.  Examples  . . . . . . . . . . . . . . . . . . . . . . . . . .   8
 7.  Security Considerations . . . . . . . . . . . . . . . . . . .   9
 8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  10
 9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  10
   9.1.  Normative References  . . . . . . . . . . . . . . . . . .  10
   9.2.  Informative References  . . . . . . . . . . . . . . . . .  11
 Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .  11
 Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  12

1. Introduction

 Commonly, several mailboxes in an IMAP message store [IMAP] have a
 special use.  For example, there can be a special-use mailbox for
 storing the user's draft messages, for keeping copies of sent
 messages, and for collecting spam messages that were classified as
 such at delivery.  The SPECIAL-USE capability [SPECIAL-USE] of the
 IMAP protocol defines mailbox attributes that identify these special
 mailboxes explicitly to the client.  This way, client configuration
 is simplified significantly.  Using the CREATE-SPECIAL-USE capability
 [SPECIAL-USE], IMAP clients can also configure these attributes
 dynamically based on user preference.
 Unlike the IMAP protocol, the Sieve mail filtering language [SIEVE]
 currently cannot freely access these special-use mailbox attributes.
 Particularly, the Sieve interpreter has no means to identify a
 mailbox with a particular special-use attribute.  This would be very
 useful, for example, to find the user's "Spam" mailbox at delivery.
 In Sieve, limited access to the special-use attributes is provided
 using the "mboxmetadata" extension [SIEVE-MAILBOX], which allows
 testing for the presence of a special-use attribute in the "/private/
 specialuse" IMAP METADATA [IMAP-METADATA] entry of a mailbox.  Still,
 not all implementers will be willing to add the complexity of the
 IMAP METADATA capability just to provide access to special-use
 attributes to the Sieve interpreter.

Bosch Standards Track [Page 2] RFC 8579 Sieve: Special-Use Mailboxes May 2019

 This document defines an extension to the Sieve mail filtering
 language that adds the ability to freely access mailbox special-use
 attributes.  It adds a test called "specialuse_exists" that checks 1)
 whether a special-use attribute is assigned for a particular mailbox
 or 2) whether any of the user's personal mailboxes have a special-use
 attribute assigned.  It also adds the ability to file messages into a
 personal mailbox identified by a particular special-use attribute
 rather than the mailbox's name.  This is achieved using the new
 ":specialuse" argument for the "fileinto" command [SIEVE].

2. Conventions Used in This Document

 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
 "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
 "OPTIONAL" in this document are to be interpreted as described in
 BCP 14 [KEYWORDS] [KEYWORDS-UPD] when, and only when, they appear in
 all capitals, as shown here.
 Conventions for notations are as described in Section 1.1 of [SIEVE],
 including use of the "Usage:" label for the definition of the action
 and the syntax of tagged arguments.
 In [IMAP] examples, "C:" and "S:" indicate lines sent by the client
 and server, respectively.  If such lines are wrapped without a new
 "C:" or "S:" label, then the wrapping is for editorial clarity and is
 not part of the command.

3. Test "specialuse_exists"

 Usage:  specialuse_exists [<mailbox: string>]
                           <special-use-attrs: string-list>
 If the "mailbox" string argument is omitted, the "specialuse_exists"
 test yields "true" if all of the following statements are true for
 each of the special-use attributes listed in the special-use-attrs
 argument:
 a.  At least one mailbox exists in the user's personal namespace
     [NAMESPACE] that has that particular special-use attribute
     assigned.
 b.  That mailbox allows the user in whose context the Sieve script
     runs to "deliver" messages into it.

Bosch Standards Track [Page 3] RFC 8579 Sieve: Special-Use Mailboxes May 2019

 If the mailbox argument is specified, the "specialuse_exists" test
 yields "true" if all of the following statements are true:
 a.  The indicated mailbox exists.
 b.  That mailbox allows the user in whose context the Sieve script
     runs to "deliver" messages into it.
 c.  That mailbox has all of the special-use attributes listed in the
     special-use-attrs argument assigned to it.
 Refer to the specification of the "mailboxexists" test in Section 3.1
 of RFC 5490 [SIEVE-MAILBOX] for a definition of when "delivery" of
 messages into a mailbox is deemed possible.

3.1. Equivalent IMAP Operations

 To clarify, the following IMAP protocol examples show a sequence of
 [IMAP] commands that a client could send to perform an assessment
 without Sieve that is equivalent to the "specialuse_exists" test.
 First, the client queries which namespaces are available using the
 NAMESPACE command [NAMESPACE]:
 C: A01 NAMESPACE
 S: * NAMESPACE (("INBOX/" "/")("Archive/" "/")) NIL (("Public/" "/"))
 S: A01 OK NAMESPACE command completed
 Subsequently, when no particular mailbox is of interest (i.e., the
 "specialuse_exists" test has no mailbox argument), the client lists
 all mailboxes with special-use attributes in the two returned
 personal namespaces (this extended LIST command requires the LIST-
 EXTENDED IMAP capability [LIST-EXTENDED]):
 C: A02 LIST (SPECIAL-USE) "" ("INBOX/*" "Archive/*")
     RETURN (SPECIAL-USE)
 S: * LIST (\Drafts) "/" INBOX/Drafts
 S: * LIST (\Trash) "/" INBOX/Trash
 S: * LIST (\Sent) "/" INBOX/Sent
 S: * LIST (\Archive) "/" Archive/Default
 S: A02 OK LIST command completed
 Finally, using the MYRIGHTS command [IMAP-ACL], the client determines
 the access rights it has for the mailbox or mailboxes that have all
 the requested attributes assigned.  This way, it can determine
 whether messages can be saved to any of those.  In this example, an
 "\Archive" special-use mailbox is sought:

Bosch Standards Track [Page 4] RFC 8579 Sieve: Special-Use Mailboxes May 2019

 C: A03 MYRIGHTS Archive/Default
 S: * MYRIGHTS Archive/Default lrwsip
 S: A03 OK Myrights completed
 The MYRIGHTS response indicates that the user has "insert" rights
 [IMAP-ACL] for the "Archive/Default" mailbox, meaning that the client
 can deliver (APPEND) messages to that mailbox and that the Sieve
 "specialuse_exists" test would yield "true" in this case.

4. ":specialuse" Argument to "fileinto" Command

 Usage:  fileinto [:specialuse <special-use-attr: string>]
                  <mailbox: string>
 Normally, the "fileinto" command delivers the message in the mailbox
 specified using its positional mailbox argument, which is the name of
 the mailbox.  However, if the optional ":specialuse" argument is also
 specified, the "fileinto" command first checks whether a mailbox
 exists in the user's personal namespace [NAMESPACE] with the
 specified special-use attribute assigned to it.  If that is the case,
 that special-use mailbox is used for delivery instead.  If there is
 no such mailbox or if the specified special-use attribute is unknown
 to the implementation in general, the "fileinto" action proceeds as
 it would without the ":specialuse" argument.
 Summarizing, if the ":specialuse" argument is specified, the
 "fileinto" command deals with two mailboxes that may or may not exist
 and may, in fact, be equal:
 o  A special-use mailbox in the user's personal namespace, which has
    at least the special-use attribute specified with the
    ":specialuse" argument assigned to it.  The name for this mailbox
    is not relevant here; it is only identified by the assigned
    special-use attribute.
 o  The default mailbox named by the positional string argument of the
    "fileinto" command, which is used when the special-use mailbox is
    not found.
 The special-use attribute specified with the ":specialuse" argument
 conforms to the "use-attr" syntax described in Section 6 of RFC 6154
 [SPECIAL-USE].  Implementations SHOULD handle an invalid special-use
 attribute in the same way as an invalid mailbox name is handled.  The
 string parameter of the ":specialuse" argument is not a constant
 string, which means that variable substitutions are allowed when the
 "variables" extension [VARIABLES] is active.  In that case, the
 syntax of the special-use attribute is only verified at runtime.

Bosch Standards Track [Page 5] RFC 8579 Sieve: Special-Use Mailboxes May 2019

 If neither the special-use mailbox nor the default mailbox exists,
 the "fileinto" action MUST proceed exactly as it does in case the
 ":specialuse" argument is absent and the mailbox named by its
 positional argument does not exist.  The various options for handling
 this situation are described in Section 4.1 of RFC 5228 [SIEVE].
 More than one mailbox in the user's personal namespace can have a
 particular special-use attribute assigned.  If one of those mailboxes
 is, in fact, the default mailbox named by the positional string
 argument of the "fileinto" command, that mailbox MUST be used for
 delivery.  If the default mailbox is not one of the options, the
 mailbox that is chosen for delivery is implementation defined.
 However, while the set of mailboxes to which the involved special-use
 attribute are assigned remains unchanged, implementations SHOULD
 ensure that the mailbox choice is made consistently, so that the same
 mailbox is used every time.  Conversely, the chosen mailbox MAY
 change once the assignments of the special-use attribute that are
 relevant for the mailbox choice are changed (usually by user
 interaction).
 If delivery to the special-use mailbox fails for reasons not relating
 to its existence, the Sieve interpreter MUST NOT subsequently attempt
 delivery in the indicated default mailbox as a fallback.  Instead, it
 MUST proceed exactly as it does in case the ":specialuse" argument is
 absent and delivery to the mailbox named by its positional argument
 fails.  This prevents the situation where messages are unexpectedly
 spread over two mailboxes in case transient or intermittent delivery
 failures occur.

4.1. Mailboxes Created Implicitly by the "fileinto" Command

 Before attempting to deliver the message into the specified mailbox,
 the "fileinto" command may implicitly create the mailbox if it does
 not exist (see Section 4.1 of RFC 5228 [SIEVE]).  This optional
 behavior can be requested explicitly using the "mailbox" extension
 [SIEVE-MAILBOX], which adds the optional ":create" argument to the
 "fileinto" command.  If the ":create" argument is specified with
 "fileinto", it instructs the Sieve interpreter to unconditionally
 create the specified mailbox if needed.  Note that the ":create"
 argument has no effect when the implicit creation of mailboxes for
 delivery is the default behavior.
 When the ":specialuse" argument is present, this behavior does not
 change; the Sieve interpreter will implicitly create the specified
 default mailbox if needed.  This need arises when both the special-
 use mailbox and the default mailbox are not found.

Bosch Standards Track [Page 6] RFC 8579 Sieve: Special-Use Mailboxes May 2019

 If the server implementation supports the CREATE-SPECIAL-USE
 capability [SPECIAL-USE] for IMAP (i.e., it allows assigning special-
 use attributes to new mailboxes), it SHOULD assign the special-use
 attribute specified with the ":specialuse" argument to the newly
 created mailbox.

4.2. Equivalent IMAP Operations

 To clarify, the following IMAP protocol examples show a sequence of
 [IMAP] commands that a client could send to perform an action without
 Sieve that is equivalent to the "fileinto" action with the
 ":specialuse" argument.  The following Sieve script is assumed:
 require "fileinto";
 require "special-use";
 fileinto :specialuse "\\Archive" "INBOX/Archive";
 First, the client proceeds as in Section 3.1 to find out whether the
 indicated special-use attribute is assigned to any mailbox in the
 user's personal namespace.  If a matching special-use mailbox is
 found, the message is delivered there using the IMAP APPEND command.
 If no matching special-use mailbox is found, the client attempts to
 deliver the message to the indicated default mailbox:
 C: A04 APPEND INBOX/Archive {309}
 S: A04 NO [TRYCREATE] Mailbox does not exist: INBOX/Archive
 In this example, the default mailbox does not exist either.  In that
 case, the client MAY create the default mailbox and assign the
 indicated special-use attribute to it:
 C: A05 CREATE INBOX/Archive (USE (\Archive))
 S: A05 OK Create completed

Bosch Standards Track [Page 7] RFC 8579 Sieve: Special-Use Mailboxes May 2019

 Finally, the client completes the delivery:
 C: A06 APPEND INBOX/Archive {309}
 S: + OK
 C: Date: Wed, 18 Jul 2018 22:00:09 +0200
 C: From: mooch@owatagu.siam.example
 C: To: Fred Foobar <foobar@Blurdybloop.example>
 C: Subject: afternoon meeting
 C: Message-Id: <Q234234-01012222@owatagu.siam.example>
 C: MIME-Version: 1.0
 C: Content-Type: text/plain; charset=UTF-8
 C:
 C: Hi Fred, do you think we can meet again at 3:30 tomorrow?
 C:
 S: A06 OK [APPENDUID 1533375901 2312] Append completed.

5. Sieve Capability Strings

 A Sieve implementation that defines the "specialuse_exists" test and
 the ":specialuse" argument for the "fileinto" command will advertise
 the capability string "special-use".

6. Examples

 The following example saves the message in the mailbox where messages
 deemed to be junk mail are held.  This mailbox is identified using
 the "\Junk" special-use attribute.  If no mailbox has this attribute
 assigned, the message is filed into the mailbox named "Spam".  If the
 mailbox named "Spam" does not exist either, the result of this Sieve
 script is implementation dependent, e.g., it may trigger an error or
 the mailbox may be created implicitly.
 require "fileinto";
 require "special-use";
 fileinto :specialuse "\\Junk" "Spam";
 The following very similar example explicitly handles the case in
 which neither a "\Junk" special-use mailbox nor the "Spam" mailbox
 exists.  In that case, a mailbox called "Spam" is created, and the
 message is stored there.  Additionally, the "\Junk" special-use
 attribute may be assigned to it.
 require "fileinto";
 require "special-use";
 require "mailbox";
 fileinto :specialuse "\\Junk" :create "Spam";

Bosch Standards Track [Page 8] RFC 8579 Sieve: Special-Use Mailboxes May 2019

 The following example is used in a Sieve script that is triggered
 from an IMAP event rather than at message delivery [IMAPSIEVE].  This
 Sieve script redirects messages to an automated recipient that
 processes junk mail if those messages are copied or moved into a
 mailbox that has the "\Junk" special-use attribute assigned.
 require "imapsieve";
 require "special-use";
 require "environment";
 require "variables";
 if environment :contains "imap.mailbox" "*" {
     set "mailbox" "${1}";
 }
 if allof(
     environment "imap.cause" "COPY",
     specialuse_exists "${mailbox}" "\\Junk") {
     redirect "spam-report@example.org";
 }

7. Security Considerations

 Security considerations are discussed in [SIEVE], [VARIABLES], and
 [SPECIAL-USE].  It is believed that this extension does not introduce
 any additional security concerns.
 Note that this specification explicitly restricts the special-use
 mailbox to the user's personal namespace.  First, this avoids the
 need to search the entire mail storage for mailboxes that have a
 particular special-use attribute assigned.  This could put undue load
 on the system, while shared special-use mailboxes are deemed of
 limited use with the currently defined special-use attributes.
 Secondly, it prevents security concerns with shared mailboxes that
 have special-use attributes assigned that apply to all users.
 Searching the entire mail storage for special-use mailboxes could
 lead to messages unexpectedly or even maliciously being filed to
 shared mailboxes.
 This restriction could be lifted for particular future special-use
 attributes, but such new attributes should have a clear application
 for shared mailboxes, and the security concerns should be considered
 carefully.

Bosch Standards Track [Page 9] RFC 8579 Sieve: Special-Use Mailboxes May 2019

8. IANA Considerations

 IANA has registered the Sieve extension specified in this document in
 the "Sieve Extensions" registry at <https://www.iana.org/assignments/
 sieve-extensions>:
    Capability name: special-use
    Description:     adds a test for checking whether an IMAP
                     special-use attribute is assigned for a
                     particular mailbox or any mailbox; also adds
                     the ability to file messages into a mailbox
                     identified solely by a special-use attribute.
    RFC number:      RFC 8579
    Contact address: Sieve discussion list <sieve@ietf.org>

9. References

9.1. Normative References

 [IMAP-METADATA]
            Daboo, C., "The IMAP METADATA Extension", RFC 5464,
            DOI 10.17487/RFC5464, February 2009,
            <https://www.rfc-editor.org/info/rfc5464>.
 [KEYWORDS] Bradner, S., "Key words for use in RFCs to Indicate
            Requirement Levels", BCP 14, RFC 2119,
            DOI 10.17487/RFC2119, March 1997,
            <https://www.rfc-editor.org/info/rfc2119>.
 [KEYWORDS-UPD]
            Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
            2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
            May 2017, <https://www.rfc-editor.org/info/rfc8174>.
 [NAMESPACE]
            Gahrns, M. and C. Newman, "IMAP4 Namespace", RFC 2342,
            DOI 10.17487/RFC2342, May 1998,
            <https://www.rfc-editor.org/info/rfc2342>.
 [SIEVE]    Guenther, P., Ed. and T. Showalter, Ed., "Sieve: An Email
            Filtering Language", RFC 5228, DOI 10.17487/RFC5228,
            January 2008, <https://www.rfc-editor.org/info/rfc5228>.
 [SIEVE-MAILBOX]
            Melnikov, A., "The Sieve Mail-Filtering Language --
            Extensions for Checking Mailbox Status and Accessing
            Mailbox Metadata", RFC 5490, DOI 10.17487/RFC5490, March
            2009, <https://www.rfc-editor.org/info/rfc5490>.

Bosch Standards Track [Page 10] RFC 8579 Sieve: Special-Use Mailboxes May 2019

 [SPECIAL-USE]
            Leiba, B. and J. Nicolson, "IMAP LIST Extension for
            Special-Use Mailboxes", RFC 6154, DOI 10.17487/RFC6154,
            March 2011, <https://www.rfc-editor.org/info/rfc6154>.
 [VARIABLES]
            Homme, K., "Sieve Email Filtering: Variables Extension",
            RFC 5229, DOI 10.17487/RFC5229, January 2008,
            <https://www.rfc-editor.org/info/rfc5229>.

9.2. Informative References

 [IMAP]     Crispin, M., "INTERNET MESSAGE ACCESS PROTOCOL - VERSION
            4rev1", RFC 3501, DOI 10.17487/RFC3501, March 2003,
            <https://www.rfc-editor.org/info/rfc3501>.
 [IMAP-ACL] Melnikov, A., "IMAP4 Access Control List (ACL) Extension",
            RFC 4314, DOI 10.17487/RFC4314, December 2005,
            <https://www.rfc-editor.org/info/rfc4314>.
 [IMAPSIEVE]
            Leiba, B., "Support for Internet Message Access Protocol
            (IMAP) Events in Sieve", RFC 6785, DOI 10.17487/RFC6785,
            November 2012, <https://www.rfc-editor.org/info/rfc6785>.
 [LIST-EXTENDED]
            Leiba, B. and A. Melnikov, "Internet Message Access
            Protocol version 4 - LIST Command Extensions", RFC 5258,
            DOI 10.17487/RFC5258, June 2008,
            <https://www.rfc-editor.org/info/rfc5258>.

Acknowledgements

 Thanks to Stan Kalisch, Barry Leiba, Alexey Melnikov, Ken Murchison,
 and Ned Freed for reviews and suggestions.
 Thanks to the authors of RFC 5490 [SIEVE-MAILBOX], from which some
 descriptive text in this document is borrowed.

Bosch Standards Track [Page 11] RFC 8579 Sieve: Special-Use Mailboxes May 2019

Author's Address

 Stephan Bosch
 Open Xchange Oy
 Lars Sonckin kaari 12
 Espoo  02600
 Finland
 Email: stephan.bosch@open-xchange.com

Bosch Standards Track [Page 12]

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