Email Greylisting: An Applicability Statement for SMTP
RFC 6647

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Internet Engineering Task Force (IETF)                      M. Kucherawy
Request for Comments: 6647                                     Cloudmark
Category: Standards Track                                     D. Crocker
ISSN: 2070-1721                              Brandenburg InternetWorking
                                                               June 2012

         Email Greylisting: An Applicability Statement for SMTP

Abstract

   This document describes the art of email greylisting, the practice of
   providing temporarily degraded service to unknown email clients as an
   anti-abuse mechanism.

   Greylisting is an established mechanism deemed essential to the
   repertoire of current anti-abuse email filtering systems.

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   http://www.rfc-editor.org/info/rfc6647.

Copyright Notice

   Copyright (c) 2012 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Kucherawy & Crocker          Standards Track                    [Page 1]
RFC 6647                       Greylisting                     June 2012

Table of Contents

   1. Introduction ....................................................3
      1.1. Background .................................................3
      1.2. Definitions ................................................4
   2. Types of Greylisting ............................................4
      2.1. Connection-Level Greylisting ...............................4
      2.2. SMTP HELO/EHLO Greylisting .................................5
      2.3. SMTP MAIL Greylisting ......................................5
      2.4. SMTP RCPT Greylisting ......................................5
      2.5. SMTP DATA Greylisting ......................................6
      2.6. Additional Heuristics ......................................7
      2.7. Exceptions .................................................7
   3. Benefits and Costs ..............................................8
   4. Unintended Consequences .........................................9
      4.1. Unintended Mail Delivery Failures ..........................9
      4.2. Unintended SMTP Client Failures ...........................10
      4.3. Address Space Saturation ..................................11
   5. Recommendations ................................................12
   6. Measuring Effectiveness ........................................13
   7. IPv6 Applicability .............................................14
   8. Security Considerations ........................................14
      8.1. Trade-Offs ................................................14
      8.2. Database ..................................................14
   9. References .....................................................15
      9.1. Normative References ......................................15
      9.2. Informative References ....................................15
   Appendix A.  Acknowledgments ......................................17

Kucherawy & Crocker          Standards Track                    [Page 2]
RFC 6647                       Greylisting                     June 2012

1.  Introduction

   Preferred techniques for handling email abuse explicitly identify
   good actors and bad actors, giving each significantly different
   service quality.  In some cases, an actor does not have a known
   reputation; this can justify providing degraded service, until there
   is a basis for providing better service.  This latter approach is
   known as "greylisting".  Broadly, the term refers to any degradation
   of service for an unknown or suspect source, over a period of time
   (typically measured in minutes or a small number of hours).  The
   narrow use of the term refers to generation of an SMTP temporary
   failure reply code for traffic from such sources.  There are diverse
   implementations of this basic concept and predictably, therefore,