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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-04     The Elvey Partnership,
                                                                   LLC
Expires: April 2007                                        A. Melnikov
                                                             Isode Ltd
                                                          October 2006



         The SIEVE mail filtering language - reject extension
                    draft-ietf-sieve-refuse-reject-04.txt

Status of this Memo

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Abstract

   This memo updates the definition the SIEVE mail filtering language (RFC
   <<3028bis>>) "reject" extension, originally defined in RFC 3028.

   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,
   Message Disposition Notifications (MDNs) and messages with
   complaints.  The original Sieve "reject" action defined in RFC 3028
   required use of MDNs for rejecting messages, thus contributing to
   the flood of Joe-job spam to victims of Joe-jobs.  This document
   updates the definition of "reject" to require rejecting messages
   during the SMTP transaction (instead of accepting them and then
   sending MDNs back to the alleged sender) wherever possible, thereby
   reducing the problem.


Table of Contents

  1. Introduction                                                   2
  2. Conventions Used in this Document                              3
  3. SIEVE "reject" extension                                       3
     3.1 Action reject                                              3
     3.2 "reject" compatibility with other actions                  7
  4. Security Considerations                                        7
  5. IANA Considerations                                            7
     5.1 reject extension registration                              7
     5.2 refuse extension registration                              8
  6. References                                                     8
     6.1 Normative References                                       8
     6.2 Informative References                                     8
  7. Acknowledgments                                                9
  8. Author's Addresses                                             9
  9. Intellectual Property Rights Statement                         9
  10. Full Copyright Statement                                     10
  11. Changes from RFC 3028                                        11
  12. Change Log                                                   11


1.   Introduction

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

   This document updates definition of the "reject" action to permit
   users to handle unwanted email in a way that is generally
   preferable to the existing 'discard' and the original '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 an
   MDN caused by the reject will be sent will often be an innocent Joe-
   job victim. The updated "reject" is less likely to result in email
   to an innocent victim, because it requires that an implemention
   refuse to accept an email for delivery instead of accepting it and
   then sending an MDN wherever possible.  Much spam is sent through
   open proxies, so SMTP level refusal reduces Joe-job bounces (AKA
   backscatter) resulting from usage of MDNs. The updated "reject"
   will also reduce Joe-jobs caused by virus self-propagation via
   emails with false sender information.  SMTP level refusal helps to
   prevent the blacklisting of sources of backscatter and conserve
   bandwidth, by reducing the number of MDNs sent. Further discussion
   highlighting the risks of generating MDNs and the benefits of
   protocol-level refusal can be found in [Joe-DoS].


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 RFC 2119.

   Conventions for notations are as in [SIEVE] section 1.1.

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


3.   SIEVE "reject" extension

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


3.1  Action reject

      Usage:   reject <reason: string>

   The "reject" action cancels the implicit keep and refuses delivery
   of a message. The reason string is a UTF-8 [UTF-8] string
   specifying the reason for refusal. How a message is refused depends
   on the capabilities of the mail component (MUA, MDA or MTA) executing
   the Sieve script. The Sieve interpreter must do one of the following
   actions, as detailed by the following priority table (items listed
   earlier take precedence). Note that if an action can not be taken or
   fails, the interpreter should try the next item in the list:

     1.   If a "reject" implementation performs a return-path verification
        and it clearly indicates that the message has a forged return-path,
        the implementation need not refuse mail delivery, but rather MAY
        accept and discard it.
     2.   Message delivery is refused by sending 5XX response code over
        SMTP [SMTP] or LMTP [LMTP]. See section 3.1.1 for more details.
     3.   Message delivery is refused by sending a non delivery report
        (DSN [DSN]). See section 3.1.2 for more details.
     4.   Message delivery is refused by sending a message disposition
        notification report (MDN). See section 3.1.3 for more details.


3.1.1 Rejecting messages at the SMTP/LMTP protocol level

   Sieve implementations that are able to reject messages at the
   SMTP/LMTP level MUST do so and SHOULD use the 550 response code. Note
   that if a message is arriving over SMTP and has multiple recipients,
   some of which have accepted the message, or the Sieve implementation
   is part of an MUA, section 3.1.2 and section 3.1.3 define how to
   reject such a message.

   Note that SMTP [SMTP] doesn't allow for non-ASCII characters in the
   SMTP response text. If non-ASCII characters appear in the "reason"
   string, they can be sent if and only if the client and the server use
   an SMTP extension that allows for transmission of non-ASCII reply
   text. (One example of such an SMTP extension is described in
   [UTF8-RESP].) In the absence of such an SMTP extension, the Sieve
   engine MUST replace any reason string containing non-ASCII characters
   with an implementation-defined ASCII-only string. Implementations
   SHOULD notify the user that such replacement took place.  Users that
   don't like this behavior should consider using "reject :exacttext" as
   described in Section 3.2, if available.

   If the "reason" string consists of multiple CRLF separated lines,
   then the reason text MUST be returned as a multiline SMTP/LMTP
   response, per [SMTP], section 4.2.1. Any line MUST NOT exceed the
   SMTP limit on the maximal line length. To make the reason string
   conform to any such limits the server MAY insert CRLFs and turn the
   response into a multiline response.

   In the following script (which assumes support for the spamtest
   [SPAMTEST] and fileinto extensions), messages that test highly
   positive for spam are refused.

      Example:
      require ["reject", "spamtest",
      "comparator-i;ascii-numeric", "fileinto"];

      if spamtest :value "ge" :comparator "i;ascii-numeric" "6" {
          reject text:
   AntiSpam engine thinks your 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-AntiSpam engine thinks your message is spam.
   S: 550-It is therefore being refused.
   S: 550 Please call 1-900-PAY-US if you want to reach us.

   If the SMTP/LMTP server supports RFC 2034 [ENHANCED-CODES] it MUST
   prepend an appropriate Enhanced Error Code to the "reason" text.
   Enhanced Error code 5.7.1 or a more generic 5.7.0 are RECOMMENDED.
   With an Enhanced Error Code, the response to DATA command in the SMTP
   example below will look like:

     S: 550-5.7.1 AntiSpam engine thinks your message is spam.
     S: 550-5.7.1 It is therefore being refused.
     S: 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.

   If a Sieve implementation that supports "reject" doesn't wish to
   immediately disclose the reason for rejection (for example that it
   detected spam), it may delay immediately sending of the 550 error
   code by sending a 4XX error code on the first attempt to receive
   the message.


3.1.2 Rejecting a message by sending a DSN

   An implementation may receive a message via SMTP that has more
   than one RCPT TO that has been accepted by the server, and at least
   one but not all of them are refusing delivery (whether the refusal
   is caused by execution of a Sieve "reject" 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
   does not apply to LMTP, as LMTP is able to reject messages on a per-
   recipient basis.

   Note that according to [DSN], Delivery Status Notifications MUST NOT
   be generated if the MAIL FROM (return-path) is empty.

3.1.3 Rejecting a message by sending an MDN

   When a Sieve engine is running inside an MUA, it has no ability to
   reject a message at the SMTP/LMTP protocol level, as final delivery
   (in the SMTP sense) is already complete. In this case the client
   should send a Message Disposition Notification [MDN] back to the
   sender. It resends the message to the sender as specified in the
   Return-Path header field, wrapping it in a "reject" form, noting that
   it was rejected by the recipient.

   MTAs and MDAs MUST NOT implement "reject" by sending MDNs, they
   SHOULD reject at the protocol level as described in section 3.1.1.
   In the following script, a message is rejected and returned to the
   sender.

   Note that according to MDN MUST NOT be generated if the MAIL FROM
   (Return-path) is empty.

   Example:
                require ["reject"];

                if header :contains "from" "coyote@desert.example.org"
   {
                reject text:
   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.

3.2  :exacttext optional argument to reject action

   SIEVE implementations that implement the :exacttext optional argument
   to the "reject" action must advertise the "rejectexact" capability in
   addition to the "reject" capability described above.

   The :exacttext argument affects how reject processing described in
   section 3.1.1 is performed. If this argument is present, and the
   SMTP client and server don't both support an SMTP extension that
   allows for transmission of non-ASCII reply text and there is
   non-ASCII text in the reason string, then the reason string MUST NOT
   be replaced with an implementation defined ASCII-only string as
   defined in 3.1.1.  Instead, the Sieve engine MUST try to generate
   DSN, in order to preserve the exact text specified in the reason
   string.

   Example:
                require ["reject", "rejectexact];

                if size :over 100K {
                reject :exacttext text:
   Your message is to big. If you want to send me a big attachement,
   put it on a public web site and send me an URL.
   .
                ;
             }

   <<Pretend that the reason string above contains some non-ASCII text>>

   NOTE: The :exacttext argument has no effect if the Sieve engine is
   running in an MUA.  The :exacttext argument also has no effect if the
   Sieve engine is running in an MTA/MDA, but both client and server
   support and negotiate use of an SMTP/LMTP extension for sending UTF-8
   responses.


3.3  "reject" compatibility with other actions

   A "reject" action cancels the implicit keep.

   Implementations MUST prohibit the execution of more than one reject
   in a SIEVE script.

   "Reject" MUST be incompatible with the "vacation" [VACATION]
   action. It is NOT RECOMMENDED that implementations permit the use of
   "reject" with actions that cause mail delivery, such as "keep",
   "fileinto", "redirect".
   Making "reject" compatible with actions that cause mail delivery
   violates the RFC 2821 principle that a message is either delivered or
   bounced back to the sender. So bouncing a message back (rejecting)
   and delivering it will make the sender believe that the message was
   not delivered.
   However, there are existing laws requiring certain organizations to
   archive all received messages, even the rejected ones. Also, it can
   be quite useful to save copies of rejected messages for later
   analysis.

   Any action that would modify the message body will not have an effect
   on the body of any message refused by "reject" using an SMTP response
   code and MUST NOT have any effect on the content of generated
   DSN/MDNs.


4.   Security Considerations

   The Introduction section talks about why rejecting messages before
   delivery is better then accepting and bouncing them.

   Security issues associated with mail auto-responders are fully
   discussed in the security consideration section of [RFC3834]. This
   document is believed not to introduce any additional security
   considerations in this general area.

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


5.   IANA Considerations

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

5.1  reject extension registration

   IANA is requested to update the registration for the SIEVE "reject"
   extension to point to this document.
   IANA is also requested to update Tim Showalter's email address to
   be
     tjs@psaux.com


5.2  refuse extension registration

   IANA is requested to remove registration of the refuse extension.
   <<Should this be taken care of by talking directly to IANA?>>


5.3  rejectexact extension registration

   IANA is requested to add the following registration to the list of
   Sieve extensions:

   Capability name: rejectexact
   Description:     adds the ':exacttext' optional argument to the
                    reject action, which instructs the Sieve engine to
                    generate Delivery Status Notifications if rejection
                    reason string contains non-ASCII text.
   RFC number:      this RFC (Sieve base spec)
   Contact address: The Sieve discussion list <ietf-mta-filters@imc.org>


6.   References

6.1  Normative References

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

   [SIEVE] Showalter, T. and P. Guenther, "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.

   [UTF-8] Yergeau, F., "UTF-8, a transformation format of ISO 10646",
   RFC 3629, November 2003.

   [VACATION] Showalter, T. and N. Freed, "Sieve Email Filtering:
   Vacation Extension", work in progress, draft-ietf-sieve-vacation-XX.txt.

6.2  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>.

   [SPAMTEST] Daboo, C., "SIEVE Email Filtering: Spamtest and
   Virustest Extensions", work in progress, draft-ietf-sieve-
   spamtestbis-XX.txt
   <<Note to the RFC editor: this reference can be safely replaced
   with RFC 3685.>>

   [RFC3834] Moore, K., "Recommendations for Automatic Responses to
   Electronic Mail", RFC 3834, August 2004.

   [UTF8-RESP] A. Melnikov (Ed.), "SMTP Language Extension",
   work in progress, draft-melnikov-smtp-lang-XX.txt


7.   Acknowledgments

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

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


8.   Author's Addresses

   Matthew Elvey
   The Elvey Partnership, LLC
   1819 Polk-ietf St Ste 133
   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


9.   Intellectual Property Rights Statement

   The IETF takes no position regarding the validity or scope of any
   Intellectual Property Rights or other rights that might be claimed
   to pertain to the implementation or use of the technology described
   in this document or the extent to which any license under such
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   it has made any independent effort to identify any such rights.
   Information on the procedures with respect to rights in RFC
   documents can be found in BCP 78 and BCP 79.

   Copies of IPR disclosures made to the IETF Secretariat and any
   assurances of licenses to be made available, or the result of an
   attempt made to obtain a general license or permission for the use
   of such proprietary rights by implementers or users of this
   specification can be obtained from the IETF on-line IPR repository
   at http://www.ietf.org/ipr.

   The IETF invites any interested party to bring to its attention any
   copyrights, patents or patent applications, or other proprietary
   rights that may cover technology that may be required to implement
   this standard.  Please address the information to the IETF at ietf-
   ipr@ietf.org.

10.  Full Copyright Statement

   Copyright (C) The Internet Society (2006).

   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
   an "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE
   REPRESENTS OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND
   THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES,
   EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT
   THE USE OF THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR
   ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A
   PARTICULAR PURPOSE.

   Acknowledgement

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


11.  Changes from RFC 3028

  Clarified that the "reject" action cancels the implicit keep.
  Extended list of allowable actions on reject to include protocol
  level message rejection and generation of DSNs.


12.  Change Log

  <<NOTE to the RFC editor: please delete this section 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".
   04   (SIEVE WG 01) Based on WGLC feedback, the refuse and the reject
       actions were merged into a single action called reject. Text
       reorganized as the result. Typos and examples corrected. Updated
       IANA registration and Security Considerations sections.
   05   (SIEVE WG 02) Copied some security considerations from Vacation
       draft. Clarified that the "reason" string is in UTF-8. Clarified
       interaction with "editheader" extension. Added text about sending
       of 4XX instead of 550. Corrected typos in several examples.
   06   (SIEVE WG 03) Explicitly list all actions incompatible with reject.
       Added two paragraphs explaining why reject SHOULD (as opposed to
       MUST/MAY) be incompatible with them. Clarified that if the reason
       string contains non-ASCII and rejection over protocol is possible,
       then the reason string MUST be replaced with an implementations
       defined ASCII-only string. Added :exacttext optional argument that
       preserves UTF-8 reason string by forcing generation of DSN.
   07   (SIEVE WG 04) Removed special handling of empty return path.
       Several editorial changes from Randy Gellens.
       Clarified :exacttext applicability, removed redundancy.  Reverted
       SHOULD NOT send MDNs back to MUST NOT send MDNs of earlier drafts
       (section 3.1.3).


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