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Versions: (draft-bosch-sieve-special-use) 00

EXTRA                                                           S. Bosch
Internet-Draft                                                Dovecot Oy
Intended status: Standards Track                      September 13, 2017
Expires: March 17, 2018


       Sieve Email Filtering: Delivering to Special-Use Mailboxes
                 draft-ietf-extra-sieve-special-use-00

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 an anonymous mailbox that has a
   particular special-use attribute assigned.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
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   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on March 17, 2018.

Copyright Notice

   Copyright (c) 2017 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



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   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.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Conventions Used in This Document . . . . . . . . . . . . . .   3
   3.  Test "specialuse_exists"  . . . . . . . . . . . . . . . . . .   3
   4.  ":specialuse" Argument to "fileinto" Command  . . . . . . . .   4
     4.1.  Interaction with ":create" Argument to "fileinto" Command   5
   5.  Sieve Capability Strings  . . . . . . . . . . . . . . . . . .   5
   6.  Examples  . . . . . . . . . . . . . . . . . . . . . . . . . .   5
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .   6
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   6
   9.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   7
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .   7
     10.1.  Normative References . . . . . . . . . . . . . . . . . .   7
     10.2.  Informative References . . . . . . . . . . . . . . . . .   8
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .   8

1.  Introduction

   Commonly, several mailboxes in an IMAP message store [IMAP] have a
   special use; e.g.  it is where the user's draft messages are stored,
   where a copy of sent messages are kept, or it is where spam messages
   are filed automatically 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 cannot find an anonymous mailbox
   that has a particular special-use attribute assigned.  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 [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.




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   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
   whether a special-use attribute is assigned for a particular mailbox
   or - if omitted - any mailbox.  It also adds the ability to file
   messages into an anonymous mailbox that has a particular special-use
   attribute assigned using a ":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", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [KEYWORDS].

   Conventions for notations are as in [SIEVE] Section 1.1, including
   use of the "Usage:" label for the definition of action and tagged
   arguments syntax.

3.  Test "specialuse_exists"

   Usage:  specialuse_exists [<mailbox: string>]
                             <special-use-flags: 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 flags listed in the "special-use-flags" argument:

   a.  at least one mailbox exists in the mail store that has that
       particular special-use flag assigned, and

   b.  that mailbox allows the user in whose context the Sieve script
       runs to "deliver" messages into it.

   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, and

   c.  that mailbox has all of the special-use flags listed in the
       "special-use-flags" argument assigned to it.

   Refer to the specification of the "mailboxexists" test in Section 3.1
   of RFC 5490 [MAILBOX] for a definition of when "delivery" of messages
   into a mailbox is deemed possible.



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

   Usage:  fileinto [:specialuse <special-use-flag: string>]
                    <mailbox: string>

   Normally, the "fileinto" command delivers the message in the mailbox
   specified using its positional mailbox argument.  However, if the
   optional ":specialuse" argument is also specified, the "fileinto"
   command first checks whether a mailbox exists with the specified
   special-use flag 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 flag 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:

   o  An anonymous special-use mailbox, which has at least the special-
      use flag specified with the ":specialuse" argument assigned to it.

   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 flag specified with the ":specialuse" argument MUST
   conform to the "use-attr" syntax described in Section 6 of RFC6154
   [MAILBOX].  Implementations SHOULD handle an invalid special-use flag
   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 flag is only verified at runtime.

   If neither the special-use mailbox nor the default mailbox exists,
   the "fileinto" action MUST proceed exactly as it does in case the
   ":specialuse" is 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 RFC5228 [SIEVE].

   More than one mailbox can have a particular special-use flag
   assigned.  In that case, the mailbox that is chosen for delivery is
   implementation-defined.  However, implementations MUST ensure that
   this choice is made consistently, so that the same mailbox is used
   every time.

   If delivery to the special-use mailbox fails for reasons not relating
   to its existence, the Sieve interpreter MUST NOT subsequently attempt



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   delivery in the indicated default mailbox as a fall-back.  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.  Interaction with ":create" Argument to "fileinto" Command

   The "mailbox" extension [MAILBOX] adds the optional ":create"
   argument to the "fileinto" command.  If the optional ":create"
   argument is specified with "fileinto", it instructs the Sieve
   interpreter to create the specified mailbox if needed, before
   attempting to deliver the message into the specified mailbox.

   When combined with the ":specialuse" argument, the ":create" argument
   instructs the Sieve interpreter to create the specified default
   mailbox if needed.  This need arises when both the special-use and
   the default mailbox are not found.

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

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".

   require "fileinto";
   require "special-use";

   fileinto :specialuse "\\Junk" "Spam";

   The following very similar example handles the case in which neither
   a "\Junk" special-use mailbox nor the "Spam" mailbox exist.  In that
   case, a mailbox called "Spam" is created, and the message is stored




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   there.  Additionally, the "\Junk" special-use attribute may be
   assigned to it.

   require "fileinto";
   require "special-use";
   require "mailbox";

   fileinto :specialuse "\\Junk" :create "Spam";

   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] and [SPECIAL-USE].
   It is believed that this extension does not introduce any additional
   security concerns.

8.  IANA Considerations

   The following template specifies the IANA registration of the Sieve
   extension specified in this document:











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      To: iana@iana.org
      Subject: Registration of new Sieve extension

      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, and it adds
                       the ability to file messages into an anonymous
                       mailbox that has a particular IMAP special-use
                       attribute assigned.
      RFC number:      this RFC
      Contact address: Sieve mailing list <sieve@ietf.org>

   This information should be added to the list of sieve extensions
   given on http://www.iana.org/assignments/sieve-extensions.

9.  Acknowledgements

   Thanks to Barry Leiba, Alexey Melnikov, Ken Murchison, and Ned Freed
   for reviews and suggestions.

10.  References

10.1.  Normative References

   [IMAP-METADATA]
              Daboo, C., "The IMAP METADATA Extension", RFC 5464, DOI
              10.17487/RFC5464, February 2009,
              <http://www.rfc-editor.org/info/rfc5464>.

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

   [MAILBOX]  Melnikov, A., "The Sieve Mail-Filtering Language --
              Extensions for Checking Mailbox Status and Accessing
              Mailbox Metadata", RFC 5490, March 2009.

   [SIEVE]    Guenther, P. and T. Showalter, "Sieve: An Email Filtering
              Language", RFC 5228, January 2008.

   [SPECIAL-USE]
              Leiba, B. and J. Nicolson, "IMAP LIST Extension for
              Special-Use Mailboxes", RFC 6154, DOI 10.17487/RFC6154,
              March 2011, <http://www.rfc-editor.org/info/rfc6154>.






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   [VARIABLES]
              Homme, K., "Sieve Email Filtering: Variables Extension",
              RFC 5229, January 2008.

10.2.  Informative References

   [IMAP]     Crispin, M., "INTERNET MESSAGE ACCESS PROTOCOL - VERSION
              4rev1", RFC 3501, DOI 10.17487/RFC3501, March 2003,
              <http://www.rfc-editor.org/info/rfc3501>.

   [IMAPSIEVE]
              Leiba, B., "Support for Internet Message Access Protocol
              (IMAP) Events in Sieve", RFC 6785, DOI 10.17487/RFC6785,
              November 2012, <http://www.rfc-editor.org/info/rfc6785>.

Author's Address

   Stephan Bosch
   Dovecot Oy
   Lars Sonckin Kaari 10
   Espoo  02600
   Finland

   Email: stephan.bosch@dovecot.fi



























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