Network Working Group                                       K. Murchison
Request for Comments: 5233                    Carnegie Mellon University
Obsoletes: 3598                                             January 2008
Category: Standards Track


              Sieve Email Filtering: Subaddress Extension

Status of This Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.

Abstract

   On email systems that allow for 'subaddressing' or 'detailed
   addressing' (e.g., "ken+sieve@example.org"), it is sometimes
   desirable to make comparisons against these sub-parts of addresses.
   This document defines an extension to the Sieve Email Filtering
   Language that allows users to compare against the user and detail
   sub-parts of an address.

Table of Contents

   1. Introduction ....................................................2
   2. Conventions Used in This Document ...............................2
   3. Capability Identifier ...........................................2
   4. Subaddress Comparisons ..........................................2
   5. IANA Considerations .............................................5
   6. Security Considerations .........................................5
   7. Normative References ............................................5
   Appendix A. Acknowledgments ........................................6
   Appendix B. Changes since RFC 3598 .................................6

1.  Introduction

   Subaddressing is the practice of augmenting the local-part of an
   [RFC2822] address with some 'detail' information in order to give
   some extra meaning to that address.  One common way of encoding
   'detail' information into the local-part is to add a 'separator
   character sequence', such as "+", to form a boundary between the
   'user' (original local-part) and 'detail' sub-parts of the address,
   much like the "@" character forms the boundary between the local-part
   and domain.

   Typical uses of subaddressing might be:

   o  A message addressed to "ken+sieve@example.org" is delivered into a
      mailbox called "sieve" belonging to the user "ken".

   o  A message addressed to "5551212#123@example.com" is delivered to
      the voice mailbox number "123" at phone number "5551212".

   This document describes an extension to the Sieve language defined by
   [RFC5228] for comparing against the 'user' and 'detail' sub-parts of
   an address.

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 [RFC2119].

3.  Capability Identifier

   The capability string associated with the extension defined in this
   document is "subaddress".

4.  Subaddress Comparisons

   Test commands that act exclusively on addresses may take the optional
   tagged arguments ":user" and ":detail" to specify what sub-part of
   the local-part of the address will be acted upon.

      NOTE: In most cases, the envelope "to" address is the preferred
      address to examine for subaddress information when the desire is
      to sort messages based on how they were addressed so as to get to
      a specific recipient.  The envelope address is, after all, the
      reason a given message is being processed by a given sieve script
      for a given user.  This is particularly true when mailing lists,

      aliases, and 'virtual domains' are involved since the envelope may
      be the only source of detail information for the specific
      recipient.

      NOTE: Because the encoding of detailed addresses are site and/or
      implementation specific, using the subaddress extension on foreign
      addresses (such as the envelope "from" address or originator
      header fields) may lead to inconsistent or incorrect results.

   The ":user" argument specifies the user sub-part of the local-part of
   an address.  If the address is not encoded to contain a detail sub-
   part, then ":user" specifies the entire left side of the address
   (equivalent to ":localpart").

   The ":detail" argument specifies the detail sub-part of the local-
   part of an address.  If the address is not encoded to contain a
   detail sub-part, then the address fails to match any of the specified
   keys.  If a zero-length string is encoded as the detail sub-part,
   then ":detail" resolves to the empty value ("").

      NOTE: If the encoding method used for detailed addresses utilizes
      a separator character sequence, and the separator character
      sequence occurs more than once in the local-part, then the logic
      used to split the address is implementation-defined and is usually
      dependent on the format used by the encompassing mail system.

   Implementations MUST make sure that the encoding method used for
   detailed addresses matches that which is used and/or allowed by the
   encompassing mail system, otherwise unexpected results might occur.
   Note that the mechanisms used to define and/or query the encoding
   method used by the mail system are outside the scope of this
   document.

   The ":user" and ":detail" address parts are subject to the same rules
   and restrictions as the standard address parts defined in [RFC5228],
   Section 2.7.4.

   For convenience, the "ADDRESS-PART" syntax element defined in
   [RFC5228], Section 2.7.4, is augmented here as follows:

         ADDRESS-PART  =/  ":user" / ":detail"

   A diagram showing the ADDRESS-PARTs of an email address where the
   detail information follows a separator character sequence of "+" is
   shown below:

          :user "+" :detail  "@" :domain
         \-----------------/
             :local-part

   A diagram showing the ADDRESS-PARTs of a email address where the
   detail information precedes a separator character sequence of "--" is
   shown below:

          :detail "--" :user  "@" :domain
         \------------------/
             :local-part

   Example (where the detail information follows "+"):

      require ["envelope", "subaddress", "fileinto"];

      # In this example the same user account receives mail for both
      # "ken@example.com" and "postmaster@example.com"

      # File all messages to postmaster into a single mailbox,
      # ignoring the :detail part.
      if envelope :user "to" "postmaster" {
          fileinto "inbox.postmaster";
          stop;
      }

      # File mailing list messages (subscribed as "ken+mta-filters").
      if envelope :detail "to" "mta-filters" {
          fileinto "inbox.ietf-mta-filters";
      }

      # Redirect all mail sent to "ken+foo".
      if envelope :detail "to" "foo" {
          redirect "ken@example.net";
      }

5.  IANA Considerations

   The following template specifies the IANA registration of the
   subaddress Sieve extension specified in this document.  This
   registration replaces that from RFC 3598:

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

   Capability name: subaddress
   Description:     Adds the ':user' and ':detail' address parts
                    for use with the address and envelope tests
   RFC number:      RFC 5233
   Contact address: The Sieve discussion list <ietf-mta-filters@imc.org>

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

6.  Security Considerations

   Security considerations are discussed in [RFC5228].  It is believed
   that this extension does not introduce any additional security
   concerns.

7.  Normative References

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

   [RFC2822]  Resnick, P., "Internet Message Format", RFC 2822, April
              2001.

   [RFC5228]  Guenther, P., Ed., and T. Showalter, Ed., "Sieve: An Email
              Filtering Language", RFC 5228, January 2008.

Appendix A.  Acknowledgments

   Thanks to Tim Showalter, Alexey Melnikov, Michael Salmon, Randall
   Gellens, Philip Guenther, Jutta Degener, Michael Haardt, Ned Freed,
   Mark Mallett, and Barry Leiba for their help with this document.

Appendix B.  Changes since RFC 3598

   o  Discussion of how the user and detail information is encoded now
      uses generic language.

   o  Added note detailing that this extension is most useful when used
      on the envelope "to" address.

   o  Added note detailing that this extension isn't very useful on
      foreign addresses (envelope "from" or originator header fields).

   o  Fixed envelope test example to only use "to" address.

   o  Replaced ":user" example with one that doesn't produce unexpected
      behavior.

   o  Refer to the zero-length string ("") as "empty" instead of "null"
      (per RFC 5228).

   o  Use only RFC 2606 domains in examples.

   o  Miscellaneous editorial changes.

Author's Address

   Kenneth Murchison
   Carnegie Mellon University
   5000 Forbes Avenue
   Cyert Hall 285
   Pittsburgh, PA  15213
   USA

   Phone: +1 412 268 2638
   EMail: murch@andrew.cmu.edu

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