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Internet Draft M. Elvey
Document: draft-elvey-refuse-sieve The Elvey Partnership, LLC
Expires: December 2004 A. Melnikov
Isode Ltd
June 2004
The SIEVE mail filtering language - refuse extension
draft-elvey-refuse-sieve-02.txt
Status of this Memo
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all provisions of Section 10 of RFC2026.
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Abstract
This memo defines the SIEVE mail filtering language [SIEVE]
"refuse" extension.
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. Discussion 2
2. Conventions Used in this Document 3
3. Introduction and Overview 3
4. SIEVE Extension 4
4.1 Action refuse 4
4.2 "refuse" compatibility with other actions 5
4.3 Explicit accomodation for servers that support Enhanced
Error Codes [ENHANCED-CODES] 5
5. Security Considerations 5
6. IANA Considerations 6
6.1 refuse extension registration 6
7. References 6
7.1 Normative References 6
7.2 Informative References 7
8. Acknowledgments 7
9. Author's Addresses 7
10. Intellectual Property Rights Statement 7
11. Full Copyright Statement 8
12. Change Log 8
1. Discussion
The SIEVE mail filtering language [SIEVE] "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. Introduction and Overview
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.
4. SIEVE Extension
This section defines the "refuse" action.
4.1 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 a 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.
SIEVE implementations that implement the "refuse" action must use
the "refuse" capability string.
4.2 "refuse" compatibility with other actions
"Refuse" cancels the implicit keep, and is incompatible with
"reject" and "discard". "Refuse" is also incompatible with
"vacation" extension [VACATION]. (It should be compatible and
incompatible with the same actions as "reject", but [SIEVE] states
"Implementations SHOULD prohibit reject when used with other
actions." However we feel that "refuse" should be permitted when
used with other actions such as "fileinto" and "redirect". This
could be useful for analyzing, tracking or reporting spam. Also,
users can use tricks (such as multiple redirects back to their own
email addresses) to get around such a prohibition anyway.)
4.3 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.
5. 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].
6. IANA Considerations
The following section provides the IANA registration for the Sieve
extensions specified in this document:
6.1 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>
7. References
7.1 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.
[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.
7.2 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
8. Acknowledgments
Thanks to Ned Freed, Cyrus Daboo, Arnt Gulbrandsen and Mark E.
Mallett for comments and corrections.
9. 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
10. Intellectual Property Rights Statement
This document is an Internet-Draft and is subject to all provisions
of Section 10 of RFC2026.
The IETF takes no position regarding the validity or scope of any
intellectual property 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 rights
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standards-related documentation can be found in BCP-11. Copies of
claims of rights made available for publication and any assurances
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to obtain a general license or permission for the use of such
proprietary rights by implementors or users of this specification
can be obtained from the IETF Secretariat.
11. Full Copyright Statement
Copyright (C) The Internet Society 2004. All Rights Reserved.
This document and translations of it may be copied and furnished to
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However, this document itself may not be modified in any way, such
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other than English.
The limited permissions granted above are perpetual and will not be
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This document and the information contained herein is provided on
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ENGINEERING TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR
IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF
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12. Change Log
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.
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