Suggested values for SMTP Enhanced Status Codes for Anti-Spam Policy
draft-macdonald-antispam-registry-01

Abstract

This document establishes a set of extended SMTP policy codes for anti-spam. It seeks to provide additional codes for error texts that currently use the extended SMTP error code 5.7.1. The anti-spam codes were determined by looking at error texts produced by major ISPs and finding commonalities. The result is a new set of error texts with associated extended SMTP error codes.

Status of this Memo

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Copyright Notice

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Table of Contents

1.  Introduction
2.  Introducing the Anti-Spam Subject
3.  Methodology of determining new detail codes
4.  IANA Considerations
    4.1.  Enumerated Status Codes
5.  Security Considerations
6.  Acknowledgements
7.  References
    7.1.  Normative References
    7.2.  Informative References
§  Author's Address

1.  Introduction

This RFC defines a set of Enhanced Status Codes [RFC3463] (Vaudreuil, “Enhanced Mail System Status Codes,” January 2003.) for SMTP related to anti-spam policy. These codes are to be registered with the IANA Mail Enhanced Status Codes registry as defined in [RFC5248] (Hansen, T., “A Registry for SMTP Enhanced Mail System Status Codes,” June 2008.). While Anti-Spam policy is inherently a local decision, assigning these codes helps troubleshoot problems and lower support costs by allowing sending administrators to resolve many problems themselves.

This document is being discussed on the SMTP mailing list, ietf-smtp@imc.org.

2.  Introducing the Anti-Spam Subject

The most common extended SMTP code assigned to anti-spam policy is 5.7.1. This is because the subject code of 7 is meant for security or policy. For anti-spam policy, the only logical detail code is 1, "Delivery not authorized, message refused". Using 5.7.1 for many different anti-spam policies weakens the usefulness of extended SMTP error codes. One of the motivations behind [RFC3463] (Vaudreuil, “Enhanced Mail System Status Codes,” January 2003.) was to re-distribute the classifications of SMTP error codes in order to provide a richer set of errors, and provide a means for machine-readable, human language independent status codes. Thus a new subject code of 8 is introduced for anti-spam policy.

3.  Methodology of determining new detail codes

All of the new detail text was gathered by surveying several existing large ISPs to see what messages were produce when presented with messages that violate their policies. An attempt was then made to coalesce the messages together into common themes. These themes where then simply assigned a detail number.

While this document provides suggested text for each detail code, alternate text can be provided if the text is in the spirit of the suggested text. This will allow sites to simply prepend the proper extended SMTP code to their existing text. Sites that are starting to implement anti-spam policy SHOULD use the text provided in this document.

While this document strives to document common anti-spam policies, it is by nature incomplete. [RFC3463] (Vaudreuil, “Enhanced Mail System Status Codes,” January 2003.) notes that new subject and detail codes will be added over time. This document is no exception to that and can be extended at future dates.

4.  IANA Considerations

IANA is directed to add the following values to the registry of Mail Enhanced Status Codes specified in [RFC5248] (Hansen, T., “A Registry for SMTP Enhanced Mail System Status Codes,” June 2008.).

The Mail Enhanced Status Codes registry will be modified to add a new subject sub-code having the value 8. This sub-code will be dedicated to anti-spam policy related SMTP errors.

All following entries will use the following common fields:

The following Subject Sub-code is defined:

4.1.  Enumerated Status Codes

The following enumerated status codes are defined.

5.  Security Considerations

As stated in [RFC3463] (Vaudreuil, “Enhanced Mail System Status Codes,” January 2003.), use of enhanced status codes may disclose additional information about how an internal mail system is implemented beyond that available through the SMTP status codes.

Some of the proposed additions to the response code list are security related. Having these registered in one place to prevent collisions will improve their value. Security error responses can leak information to active attackers (e.g., the distinction between "user not found" and "bad password" during authentication). Documents defining security error codes should make it clear when this is the case, so that SMTP server software subject to such threats can provide appropriate controls to restrict exposure.

6.  Acknowledgements

A special thanks Tony Hansen, Murray S. Kucherawy, sm and Hector Santos for helpful comments.

7.  References

7.1. Normative References

7.2. Informative References

Author's Address