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Versions: 00 01 02 03 04 05 06 07 08 09 RFC 5429

Internet Draft                                                M. Elvey
Document: draft-ietf-sieve-refuse-reject-00  The Elvey Partnership,LLC
Expires: November 2005                                     A. Melnikov
                                                             Isode Ltd
                                                              May 2005



   The SIEVE mail filtering language - reject and refuse extensions
                    draft-ietf-sieve-refuse-reject

Status of this Memo

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Abstract

   This memo defines the SIEVE mail filtering language [SIEVE]
   "reject" and "refuse" extensions.

   A Joe-job is a spam run forged to appear as though it came from an
   innocent party, who is then generally flooded by the bounces, MDNs
   and messages with complaints.  With the Sieve "reject" action, MDNs
   contribute to the flood of Joe-job spam to victims of Joe-jobs;
   SMTP level refusals usually don't.  With "refuse", Sieve gains the
   ability to simply not accept an email during the SMTP transaction
   (instead of accepting it and then sending an MDN [MDN] back to the
   alleged sender using "reject").


Table of Contents

  1. Introduction                                                   4
  2. Conventions Used in this Document                              4
  3. Discussion of finer points                                     4
  4. SIEVE "reject" extension                                       5
     4.1 Action reject                                              5
     4.2 "reject" compatibility with other actions                  6
  5. SIEVE "refuse" extension                                       6
     5.1 Action refuse                                              6
     5.2 "refuse" compatibility with other actions                  7
     5.3 Explicit accomodation for servers that support Enhanced
     Error Codes [ENHANCED-CODES]                                   7
  6. Security Considerations                                        8
  7. IANA Considerations                                            8
     7.1 reject extension registration                              8
     7.2 refuse extension registration                              8
  8. References                                                     9
     8.1 Normative References                                       9
     8.2 Informative References                                     9
  9. Acknowledgments                                                9
  10. Author's Addresses                                           10
  11. Intellectual Property Rights Statement                       10
  12. Full Copyright Statement                                     11
  13. Changes from RFC 3028                                        11
  14. Change Log                                                   11



1.   Introduction

   The SIEVE mail filtering language [SIEVE] "reject" action allows
   users to refuse delivery of a message by sending an [MDN]. This
   action was originally defined in RFC 3028 [SIEVE].

   The "refuse" extension, if supported, permits users to handle
   unwanted email in a way that is sometimes preferable to the
   existing 'discard' and 'reject' capabilities.  When a spam-
   detection system suspects a message is spam, but isn't certain,
   discarding the email is considered too risky for some users, for
   example, those who receive sales leads by email. They are willing
   to use the reject command. Users are willing to reject but not
   discard because the sender of an email incorrectly marked as spam
   will receive a notification that the email was refused, and will
   likely try again to contact the intended recipient, perhaps via
   another method of communication. Unfortunately, this usage is
   problematic, because in the usual case, the email is indeed spam,
   and the alleged sender to whom the MDN caused by the reject will be
   sent will often be an innocent Joe-job victim.  "Refuse" is
   intended to be superior to "reject" because it will be less likely
   to result in email to an innocent victim.  "Refuse" refuses to
   accept an email for delivery instead of accepting it and then
   sending an MDN.  Much spam is sent through open proxies, so
   "refuse" reduces Joe-job bounces resulting from usage of reject.
   "Refuse" will also reduce Joe-jobs caused by virus self-propagation
   via emails with false sender information.  "Refuse" may conserve
   bandwidth, by reducing the number of MDNs sent. Further discussion
   highlighting the risks of "reject" and the benefits of "refuse" can
   be found in [Joe-DoS].


2.   Conventions Used in this Document

   Conventions for notations are as in [SIEVE] section 1.1, including
   use of [KEYWORDS].

   This document does not attempt to define what exactly constitutes a
   spam or virus containing email or how it should be identified, or
   what actions should be taken when detected.


3.   Discussion of finer points

   The "refuse" action MUST refuse to accept an email for delivery at
   the SMTP/LMTP level by returning a 5XX reply code, instead of
   sending an MDN as required by the "reject" action, other than for
   the two exceptions specified below.  A SIEVE implementation that
   cannot do so MUST NOT claim to support the refuse extension.

   There is an exception when a message has multiple valid recipients,
   and at least one but not all of them are refusing delivery (whether
   the refusal is caused by execution of a Sieve "refuse" or for
   another reason). In this case, the server MUST accept the message
   and generate DSNs for all recipients that are refusing it. Note
   that this exception only applies to SMTP, as LMTP is able to reject
   messages on a per-recipient basis.

   If a "refuse" implementation performs a return-path verification
   and it clearly indicates that the message has a forged return-path,
   the implementation need not refuse to accept the mail, but rather
   MAY accept and discard it.

   The "reject" action is defined so that it can be used by
   implementations unable to implement "refuse" (i.e. by MUAs) or for
   backwards compatibility with scripts based on RFC3028.


4.   SIEVE "reject" extension

   SIEVE implementations that implement the "reject" action must use
   the "reject" capability string.


4.1  Action reject

      Syntax:   reject <reason: string>

   The "reject" action refuses delivery of a message by sending back
   an [MDN] to the sender. The "reject" action also cancels the
   implicit keep.  It resends the message to the sender, wrapping it
   in a "reject" form, noting that it was rejected by the recipient.
   In the following script, a message is rejected and returned to the
   sender.

   Example:
             require ["reject"]

             if header :contains "from" "coyote@desert.example.org" {
                reject "I am not taking mail from you, and I don't
                want your birdseed, either!";
             }

   A reject message MUST take the form of a failure MDN as specified
   by [MDN]. The human-readable portion of the message, the
   first component of the MDN, contains the human readable message
   describing the error, and it SHOULD contain additional text
   alerting the original sender that mail was refused by a filter.
   This part of the MDN might appear as follows:

   ------------------------------------------------------------
   The message was refused by the recipient's mail filtering program.
   The reason
   given was as follows:

   I am not taking mail from you, and I don't want your birdseed,
   either!
   ------------------------------------------------------------

   The MDN action-value field as defined in the MDN specification MUST
   be "deleted" and MUST have the MDN-sent-automatically and automatic-
   action modes set.


4.2  "reject" compatibility with other actions

   A "reject" action cancels the implicit keep.

   Implementations MUST prohibit more than one reject in a SIEVE
   script. "Reject" is also incompatible with the "refuse" and
   "vacation" [VACATION] extensions.

   Implementations SHOULD prohibit reject when used with other
   actions.


5.   SIEVE "refuse" extension

   SIEVE implementations that implement the "refuse" action must use
   the "refuse" capability string.


5.1  Action refuse

      Syntax:   refuse <reason: string>

   The "refuse" action refuses delivery of a message by sending back
   the 550 SMTP response code to an SMTP client.

   This extension can be only supported by a Sieve implementation
   running in an MTA.

   Note that SMTP [SMTP] doesn't allow for non-ASCII characters in
   SMTP response text. It is an error for non-ASCII characters to
   appear in the "reason" string (unless the client and the server use
   an SMTP extension that allows for transmission of non-ASCII reply
   text; such an extension is not known to the authors).

   If the "reason" string is multiline, than the reason text MUST be
   returned as a multiline SMTP/LMTP response, per [SMTP], section
   4.2.1.

   In the following script (which assumes support for the spamtest
   extension), messages that test highly positive for spam are
   refused.

      Example:
      require ["refuse", "spamtest"]

      if spamtest :value "ge" :comparator "i;ascii-numeric" "6" {
                   refuse text:
   SpamAssassin thinks the message is spam.
   It is therefore being refused.
   Please call 1-900-PAY-US if you want to reach us.
   .
                            ;
      elsif spamtest :value "ge" :comparator "i;ascii-numeric" "4" {
                   fileinto "Suspect";
                  }

  The following excerpt from an SMTP session shows it in action.

  C: DATA
  S: 354 Send message, ending in CRLF.CRLF.
   ...
  C: .
  S: 550-SpamAssassin thinks the message is spam.
  S: 550-It is therefore being refused.
  S: 550 Please call 1-900-PAY-US if you want to reach us.


5.2  "refuse" compatibility with other actions

   "Refuse" cancels the implicit keep, and is incompatible with
   "reject" and "discard". "Refuse" is also incompatible with the
   "vacation" [VACATION] action. Any action that would modify the
   message body will necessarily have no effect on the body of any
   message refused by "refuse" using the 550 SMTP response code.
   If a script attempts to "refuse" the same message more than once,
   the implementation may ignore the later attempts or consider it
   an error."

5.3  Explicit accomodation for servers that support Enhanced Error
    Codes [ENHANCED-CODES]

   This section only concerns implementations that support Enhanced
   Error Codes.

   If the server supports RFC 2034 [ENHANCED-CODES] it MUST select an
   appropriate Enhanced Error Code (e.g. 5.7.1 or a more generic
   5.7.0) and prepend it to the "reason" text. I.e. on such an
   implementation, the example in section 4.1 would show up in SMTP
   as:

     550-5.7.1 SpamAssassin thinks the message is spam.
     550-5.7.1 It is therefore being refused.
     550 5.7.1 Please call 1-900-PAY-US if you want to reach us.

   if the server selected "5.7.1" as appropriate.


6.   Security Considerations

   The "refuse" extension does not raise any security considerations
   that are not present in the base [SIEVE] protocol, and these issues
   are discussed in [SIEVE].


7.   IANA Considerations

   The following section provides the IANA registrations for the Sieve
   extensions specified in this document:

7.1  reject extension registration

   IANA is requested to update the registration for the SIEVE "reject"
   and "refuse" extensions to point to this document.

   <<Update Tim's email address as well?>>


7.2  refuse extension registration

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

   Capability name: refuse
   Capability keyword: refuse
   Capability arguments: N/A
   Standards Track/IESG-approved experimental RFC number: this RFC
   Person and email address to contact for further information:

       Matthew Elvey
       The Elvey Partnership, LLC
       3042 Sacramento-ietf St Ste 04
       San Francisco, CA
       U.S.A.

       <mailto:sieve3@matthew.elvey.com>


8.   References

8.1  Normative References

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

   [SIEVE] Showalter, "Sieve: A Mail Filtering Language", RFC 3028,
   January 2001.

   Guenther, P., "Sieve: An Email Filtering Language", Work-in-
   progress, draft-ietf-sieve-3028bis-XX.txt

   [SMTP] Klensin, J. (Editor), "Simple Mail Transfer Protocol", AT&T
   Laboratories, RFC 2821, April 2001.

   [LMTP] Myers, J., "Local Mail Transfer Protocol", Carnegie-Mellon
   University, RFC 2033, October 1996.

   [DSN] Moore , K., Vaudreuil, G., "An Extensible Message Format for
   Delivery Status Notifications", University of Tennessee, Lucent
   Technologies, RFC 3464, January 2003.

   [MDN] Fajman, R., "An Extensible Message Format for Message
   Disposition Notifications", National Institutes of Health, RFC
   2298, March 1998.

   [ENHANCED-CODES] Freed, N., "SMTP Service Extension for Returning
   Enhanced Error Codes", Innosoft, RFC 2034, October 1996.


8.2  Informative References

   [Joe-DoS] Stefan Frei, Ivo Silvestri, Gunter Ollmann, "Mail Non
   Delivery Message DDoS Attacks", 5 April 2004",
   <http://www.techzoom.net/paper-mailbomb.asp>.


9.   Acknowledgments

   Thanks to Ned Freed, Cyrus Daboo, Arnt Gulbrandsen, Kristin Hubner,
   Mark E. Mallett and Philip Guenther for comments and corrections.

   The authors gratefully acknowledge the extensive work of Tim
   Showalter as the author of the RFC 3028, which originally defined
   "reject".


10.  Author's Addresses

   Matthew Elvey
   The Elvey Partnership, LLC
   3042 Sacramento-ietf St Ste 04
   San Francisco, CA
   U.S.A.

   Email: sieve3@matthew.elvey.com


   Alexey Melnikov
   Isode Limited
   5 Castle Business Village
   36 Station Road
   Hampton, Middlesex, TW12 2BX
   UK

   Email: Alexey.Melnikov@isode.com


11.  Intellectual Property Rights Statement

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12.  Full Copyright Statement

   Copyright (C) The Internet Society (2005).

   This document is subject to the rights, licenses and restrictions
   contained in BCP 78, and except as set forth therein, the authors
   retain all their rights.

   This document and the information contained herein are provided on
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   Acknowledgement

   Funding for the RFC Editor function is currently provided by the
   Internet Society.


13.  Changes from RFC 3028

  Clarified that the "reject" action cancels the implicit keep.


14.  Change Log

  <<Note that this section will be deleted before publication.>>

   00   First formal draft.
   01   Explicit RFC 2034 support, disallow "refuse" in MUAs, typos
        corrected, clarifications, etc.
   02   Many insubstantial editorial changes (mostly rewording text for
        readability). Added text regarding non-ASCII characters in the
        refuse "reason" string. Added an exception allowing return-path
        forgery to justify discarding a message.
   03   (Renamed to be SIEVE WG 00) - Updated boilerplate, added reject
        action from the base spec, acknowledged Tim as the author of
        "reject".


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