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

Sieve Working Group                                        A. Stone, Ed.
Internet-Draft                                               Serendipity
Obsoletes: 3028 (if approved)
Updates: 5228 (if approved)                            November 17, 2008
Intended status: Standards Track
Expires: May 21, 2009


      Sieve Email Filtering: Reject and Extended Reject Extensions
                   draft-ietf-sieve-refuse-reject-09

Status of this Memo

   By submitting this Internet-Draft, each author represents that any
   applicable patent or other IPR claims of which he or she is aware
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   This Internet-Draft will expire on May 21, 2009.

Abstract

   This memo updates the definition of the Sieve mail filtering language
   "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 automated bounces,
   Message Disposition Notifications (MDNs), and personal 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.




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   This memo updates the definition of the "reject" action to allow
   messages to be refused during the SMTP transaction, and defines the
   "ereject" action to require messages to be refused during the SMTP
   transaction, if possible.

   The "ereject" action is intended to replace the "reject" action
   wherever possible.  The "ereject" action is similar to "reject", but
   will always favor protocol level message rejection.


Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
     1.1.  Conventions Used in This Document  . . . . . . . . . . . .  3
   2.  Sieve 'reject' and 'ereject' Extentions  . . . . . . . . . . .  4
     2.1.  Action ereject . . . . . . . . . . . . . . . . . . . . . .  4
       2.1.1.  Rejecting a message at the SMTP/LMTP protocol level  .  5
       2.1.2.  Rejecting a message by sending a DSN . . . . . . . . .  5
     2.2.  Action reject  . . . . . . . . . . . . . . . . . . . . . .  6
       2.2.1.  Rejecting a message by sending an MDN  . . . . . . . .  7
     2.3.  Silent upgrade from reject to ereject  . . . . . . . . . .  8
     2.4.  Compatibility with other actions . . . . . . . . . . . . .  8
     2.5.  Details of protocol level refusal  . . . . . . . . . . . .  9
   3.  Changes from RFC 3028  . . . . . . . . . . . . . . . . . . . . 11
   4.  Security Considerations  . . . . . . . . . . . . . . . . . . . 11
   5.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 11
     5.1.  reject extension registration  . . . . . . . . . . . . . . 11
     5.2.  ereject extension registration . . . . . . . . . . . . . . 12
   6.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 12
     6.1.  Normative References . . . . . . . . . . . . . . . . . . . 12
     6.2.  Informative References . . . . . . . . . . . . . . . . . . 13
   Appendix A.  Acknowledgements  . . . . . . . . . . . . . . . . . . 13
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 13
   Intellectual Property and Copyright Statements . . . . . . . . . . 15

















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1.  Introduction

   The Sieve mail filtering language, as originally defined in RFC 3028
   [SIEVE], specified that the "reject" action shall discard a message
   and send a Message Disposition Notification [MDN] to the envelope
   sender along with an explanatory message.  The Sieve mail filtering
   language, as updated in RFC 5228 [SIEVEBIS], does not define any
   reject action, hence the purpose of this document.

   This document updates the definition of the "reject" action to permit
   refusal of the message during the SMTP transaction, if possible, and
   defines a new "ereject" action to require refusal of the message
   during the SMTP transaction, if possible.

   An important goal of this document is to reduce the risk of Sieve
   scripts being used to perpetrate "Joe-job" spam runs, where the MDN
   sent notifying the sender of a message of its non-delivery is in fact
   sent to an innocent third-party.  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.  By
   rejecting the message at the protocol level, it is less likely that
   an MDN will be needed, and so less likely that an MDN will be
   misdirected at an innocent third-party.

   Implementations are further encouraged to use spam-detection systems
   to determine the level of risk associated with sending an MDN, and
   this document allows implementations to silently drop the MDN if the
   rejected message is deemed to be likely spam.

   This document also describes how to use reject/ereject at varying
   points in the email stack: Mail Transfer Agent (MTA), Mail Delivery
   Agent (MDA), and Mail User Agent (MUA).  See [EMAIL-ARCH] for a
   comprehensive discussion of these environments.

   In general, an MDN is generated by an MUA, and can be used to
   indicate the status of a message with respect to its recipient, while
   a DSN [DSN] is generated by an MTA, and can be used to indicate
   whether or not a message was received and delivered by the mail
   system.

   Further discussion highlighting the risks of generating MDNs and the
   benefits of protocol-level refusal can be found in [Joe-DoS].

1.1.  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].



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   Conventions for notations are as in RFC 5228 [SIEVEBIS] Section 1.1.

   This document does not attempt to define spam or how it should be
   identified, nor to define an email virus or how it should be
   detected.  Implementors are advised to follow best practices and keep
   abreast of current research in these fields.


2.  Sieve 'reject' and 'ereject' Extentions

2.1.  Action ereject

   Usage: ereject <reason: string>

   Sieve implementations that implement the "ereject" action must use
   the "ereject" capability string.

   The "ereject" 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 (MDA or MTA) executing the Sieve
   script.  The Sieve interpreter MUST carry out one of the following
   actions (listed in order from most to least preferred), MUST carry
   out the most preferable action possible, and MUST fall back to lesser
   actions if a preferred action fails.

   1.  Refuse message delivery by sending a 5XX response code over SMTP
       [SMTP] or LMTP [LMTP].  See Section 2.1.1 for more details.

   2.  Send a non-delivery report to the envelope sender ([REPORT]
       [DSN]), unless the envelope sender address is determined to be a
       forged or otherwise invalid address.

   Note that determination of whether or not an envelope sender is a
   forgery may be performed by site-specific and implementation-specific
   heuristic techniques, such as "return-path verification", details of
   which are outside the scope of this document.  Implementations SHOULD
   log instances when a non-delivery report is not sent and the reason
   for not sending the report (e.g. content was spam, return-path
   invalid, etc.).

   The ereject action MUST NOT be available in environments that do not
   support protocol level rejection, e.g. an MUA, and MUST be available
   in all other environments that support the reject action.







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       Example:
               require ["ereject"];

               if address "from" "someone@example.com" {
                   ereject "I no longer accept mail from this address";
               }

2.1.1.  Rejecting a message 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 whom have accepted the message, Section 2.1.2 defines how to
   reject such a message.

   The risk that these actions will generate blowback spam are minimized
   but cannot be eliminated completely even in the case of ereject, so
   caution is advised when using these actions to deal with messages
   determined to be spam.

   Note that SMTP [SMTP] does not allow non-ASCII characters in the SMTP
   response text.  If non-ASCII characters appear in the "reason"
   string, they can be sent at the protocol level 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 being
   sent at the protocol level and containing non-ASCII characters with
   an implementation-defined ASCII-only string.

   Users who don't like this behavior should consider using the "reject"
   action described in Section 2.2, if available.

   See Section 2.5 for the detailed instructions about performing
   protocol level rejection.

2.1.2.  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 a Sieve "ereject" action or for some other 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.  (However, the LMTP client may then have no choice
   but to generate a DSN to report the error, which may result in
   blowback spam.)



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

   The DSN message MUST follow the requirements of [DSN] and [REPORT]
   The action-value field defined in [DSN], Section 2.3.3, MUST contain
   the value "failed".  The human-readable portion of the non-delivery
   report MUST contain the reason string from the "ereject" action and
   SHOULD contain additional text alerting the apparent original sender
   that the message was refused by an email filter.  This part of the
   report might appear as follows:

   ------------------------------------------------------------
   Your 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!
   ------------------------------------------------------------

2.2.  Action reject

   This section updates the definition of the reject action in Section
   4.1 of RFC 3028 and is an optional extension to [SIEVEBIS].

          Usage:   reject <reason: string>

   Sieve implementations that implement the "reject" action must use the
   "reject" capability 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.  Unlike the "ereject" action described above,
   this action would always favor preserving the exact text of the
   refusal reason.  Typically the "reject" action refuses delivery of a
   message by sending back an MDN to the sender (see Section 2.2.1).
   However implementations MAY refuse delivery over SMTP/LMTP protocol
   (as detailed in Section 2.5), if and only if all of the following
   conditions are true:

   1.  The reason string consists of only US-ASCII characters
         or
       The reason string contains non-US-ASCII and both client and
       server support and negotiate use of an SMTP/LMTP extension for
       sending UTF-8 responses.







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   2.  LMTP protocol is used
         or
       SMTP protocol is used and the message has a single recipient
         or
       SMTP protocol is used, the message has multiple recipients, and
       all of them refused message delivery (whether using Sieve or
       not).


       Example:
               require ["reject"];

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

   (Pretend that the reason string above contains some non-ASCII text.)

   Implementations may use techniques as described in Section 2.1 to
   determine if a non-delivery report should not be sent to a forged
   sender.  Implementations SHOULD log instances when a non-delivery
   report is not sent and the reason for not sending the report.

2.2.1.  Rejecting a message by sending an MDN

   The reject action resends the received message to the envelope sender
   specified by the MAIL FROM (or Return-Path) address, wrapping it in a
   "reject" form, explaining that it was rejected by the recipient.

   Note that according to [MDN], Message Disposition Notifications MUST
   NOT be generated if the MAIL FROM (or Return-Path) is empty.

   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
   apparent original sender that mail was refused by an email filter.

   The MDN disposition-field as defined in the MDN specification MUST be
   "deleted" and MUST have the "MDN-sent-automatically" and "automatic-
   action" modes set (see Section 3.2.6 of [MDN]).

   In the following script, a message is rejected and returned to the
   sender.



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       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!"
       .
                   ;
               }

   For this script, the first 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!
   ------------------------------------------------------------

2.3.  Silent upgrade from reject to ereject

   Implementations MUST NOT silently upgrade reject actions to ereject
   actions in a Sieve script, because this might lead to unpleasant
   changes of behavior not expected by the script owner.

   User interfaces that present a generic rejection option, and generate
   Sieve script output, MAY switch from generating reject to ereject
   actions, so long as doing so does not create a confusing change for
   the script owner.

   Script generators SHOULD ensure that a rejection action being
   executed as a result of an anti-spam/anti-virus positive test be done
   using the ereject action, as it is more suitable for such rejections.

   Script generators MAY automatically upgrade scripts that previously
   used the reject action for anti-spam/anti-virus related rejections.
   Note that such generators MUST make sure that the target environment
   can support the ereject action.

2.4.  Compatibility with other actions

   This section applies equally to "reject" and "ereject" actions.  All
   references to the "reject" action in this section can be replaced
   with the "ereject" action.

   A "reject" action cancels the implicit keep.



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   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 [SMTP] 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.

2.5.  Details of protocol level refusal

   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.















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

               if spamtest :value "ge"
                           :comparator "i;ascii-numeric" "6" {
                   ereject 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 "ereject" does not 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.






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3.  Changes from RFC 3028

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

   Added the "ereject" action that is similar to "reject", but will
   always favor protocol level message rejection.


4.  Security Considerations

   The Introduction to this document discusses why rejecting messages
   before delivery is better than accepting and bouncing them.

   While the details of techniques that can be used to determine when to
   silently drop a non-delivery report are outside the scope of this
   document, the explicit permission this document gives to take such
   action may enable denial of service situations.  Techniques such as
   spam-checking, return-path verification, and others, can and do have
   false-positives.  Care should be exercised to prevent the loss of
   legitimate messages by failing to notify the sender of non-delivery.

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

   The "ereject" extension does not raise any other security
   considerations that are not already present in the base [SIEVE]
   specification, 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 as detailed below:

   Capability name: reject
   Description:     adds the "reject" action for refusing delivery
                    of a message.  The exact reason for refusal is
                    conveyed back to the client.
   RFC number:      this RFC



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   Contact address: the Sieve discussion list <ietf-mta-filters@imc.org>

5.2.  ereject extension registration

   IANA is requested to replace the preliminary registration of the
   Sieve refuse extension with the following registration:

   Capability name: ereject
   Description:     adds the 'ereject' action for refusing delivery
                    of a message. The refusal should happen as early
                    as possible (e.g. at the protocol level) and might
                    not preserve the exact reason for refusal if it
                    contains non-US-ASCII text.
   RFC number:      this RFC
   Contact address: the Sieve discussion list <ietf-mta-filters@imc.org>


6.  References

6.1.  Normative References

   [DSN]      Moore, K. and G. Vaudreuil, "An Extensible Message Format
              for Delivery Status Notifications", RFC 3464,
              January 2003.

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

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

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

   [MDN]      Hansen, T. and G. Vaudreuil, "Message Disposition
              Notification", RFC 3798, May 2004.

   [REPORT]   Vaudreuil, G., "The Multipart/Report Content Type for the
              Reporting of Mail System Administrative Messages",
              RFC 3462, January 2003.

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

   [SMTP]     Klensin, J., "Simple Mail Transfer Protocol", RFC 2821,



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              April 2001.

   [UTF-8]    Yergeau, F., "UTF-8, a transformation format of ISO
              10646", STD 63, RFC 3629, November 2003.

   [VACATION]
              Showalter, T. and N. Freed, "Sieve Email Filtering:
              Vacation Extension", RFC 5230, January 2008.

6.2.  Informative References

   [EMAIL-ARCH]
              Crocker, D., "Internet Mail Architecture",
              draft-crocker-email-arch-11 (work in progress),
              October 2008.

   [Joe-DoS]  Frei, S., Silvestri, I., and G. Ollman, "Mail Non-Delivery
              Notice Attacks", April 2004, <http://www.techzoom.net/
              papers/mail_non_delivery_notice_attacks_2004.pdf>.

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

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

   [SPAMTEST]
              Daboo, C., "Sieve Email Filtering: Spamtest and Virustest
              Extensions", RFC 5235, January 2008.

   [UTF8-RESP]
              Melnikov, A., "SMTP Language Extension",
              draft-melnikov-smtp-lang-07 (work in progress), June 2007.


Appendix A.  Acknowledgements

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







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Authors' Addresses

   Aaron Stone (editor)
   Serendipity
   260 El Verano Ave
   Palo Alto, CA  94306
   USA

   Email: aaron@serendipity.palo-alto.ca.us


   Matthew Elvey
   The Elvey Partnership, LLC
   1819 Polk Street, Suite 133
   San Francisco, CA  94109
   USA

   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























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Full Copyright Statement

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