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This appendix lists useful online resources for further information about spam, spam-filtering, SpamAssassin, each of the mail transport agents discussed in this book, and several other SpamAssassin-related software packages.


General Spam Resources

"Help! I've been spammed! What do I do?"—originally written by Chris Lewis and maintained by Greg Byshenk—is an helpful (if dated) guide to spam and spam-prevention for the beginner. Find it at

Internet Request For Comments (RFC) documents describe proposed standards for the Internet. You can get RFCs from Some notable RFCs related to spam and spam filtering include

RFC 2822: Internet Message Format
The basic document that describes the formatting of email messages.
RFC 2821: Simple Mail Transfer Protocol
Explains SMTP, the protocol used to transfer email from system to system.
RFC 2505: Anti-spam Recommendations for Internet MTAs
Describes a set of best practices for mail servers.

The SPAM-L FAQ, maintained by Doug Muth at, provides information about the SPAM-L mailing list, one of the oldest discussion forums for spam fighters. is a long-standing site with information for antispam advocates and system adminstrators.

The Coalition Against Unsolicited Commercial Email (CAUCE) has a web site at CAUCE focuses primarily on advocacy and legislation.

The groups in the Usenet hierarchy are devoted to discussing and reporting Net abuse, including spam (see particularly

Spam-Filtering provides a long list of DNSBLs that may be suitable for spam-filtering. It includes hit rates (but not false positive rates) against its own recently collected spam corpus for each DNSBL.

Vipul's Razor, Pyzor, and DCC are collaborative spam-filtering clearinghouses that can be consulted by SpamAssassin. Vipul's Razor is available at Pyzor is at DCC is at

DSPAM is a spam-filtering system using statistical-algorithmic hybrid filtering—filters that are trained like SpamAssassin's Bayesian classifier. Find it at

CRM114, the Controllable Regex Multilator, is a spam-filtering system based on learning regular expressions. Download it at


The home page for SpamAssassin itself is, of course, The site contains links to the spamassassin-users and spamassassin-dev mailing lists.

SAProxy Pro, a commercial client-side SpamAssassin proxy, is available from

Mail Transport Agents

In this book, I describe how to use SpamAssassin in conjunction with several MTAs. The following sections point you to more information about each of those agents.


Sendmail has two primary web sites. The open source version maintained by the Sendmail consortium can be found at Sendmail's commercial face is

Sendmail's own antispam provisions are documented at If you're not a Sendmail guru, pick up Bryan Costales' book Sendmail (O'Reilly), the Sendmail bible. If you are a Sendmail guru, you probably already have a copy! Another good source is Frederick Avolio and Paul Vixie's Sendmail: Theory and Practice (Digital Press).

Sendmail's filtering architecture, milter, has led to the development of many filtering tools. The web site is the most comprehensive catalog of such filters.


The home page for the Postfix MTA is

Two useful manuals for Postfix are Postfix: The Definitive Guide by Kyle Dent (O'Reilly) and The Book of Postfix by Ralf Hildebrandt and Patrick Koetter (No Starch Press).


The home page for the qmail MTA is The netqmail distribution of qmail includes the QMAILQUEUE patch, which is used by most SpamAssassin-integration solutions.

The online book Life with qmail ( provides excellent documentation for qmail.


The home page for the Exim MTA is The site includes a link to the Exim specification, which serves as the manual for the MTA. Those preferring a real book should purchase Philip Hazel's The Exim SMTP Mail Server: Official Guide for Release 4 (UIT Cambridge).

Related Mail Tools

The following sections point to information about various other mail tools mentioned in this book.


procmail is a popular and powerful Unix filtering program that acts as a full-featured local email delivery agent. It's available at


MIMEDefang is a Sendmail milter application written in Perl that provides a framework for mail content scanning, including virus-scanning, spam-checking with SpamAssassin, and MIME validation. It's available at Versions after 2.42 support SpamAssassin 3.0.


amavisd-new is a high-performance daemonized content scanner, designed for use with Postfix, but it also supports Sendmail (including a milter version) and Exim. It's available at Don't confuse amavisd-new with amavis, amavis-perl, amavisd, or amavis-ng, all of which are other content scanners, most not actively in development, that now share little code in common with amavisd-new.


sa-exim is a local_scan.c replacement for Exim that provides SpamAssassin message-scanning during a SMTP transaction. It offers many options for how to handle spam that it detects, including teergrubing. sa-exim's home page is


exiscan-acl is a patch for Exim 4 that adds new content-scanning ACL directives to Exim. These directives can be used to invoke SpamAssassin on messages during a SMTP transaction. Some prepackaged Exim distributions already have this patch added. You can download it at


qmail-scanner is a content scanner for qmail that can run SpamAssassin on messages early in the delivery process. It contains its own implementation of spamc for faster checking. Find it at

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