Mail Avenger features
Why use the avenger SMTP server?

Mail avenger is a highly-configurable, MTA-independent SMTP (simple mail transport protocol) server. It allows you to reject spam during mail transactions, before spooling messages in your local mail queue. You can specify site-wide default policies for filtering mail, but individual users can also craft their own policies by creating avenger scripts in their home directories.

Compared to traditional (.forward, .qmail, etc.) spam filtering, filtering during an SMTP transaction gives you more options. For instance, you can reject mail with an SMTP error code, causing a bounce only if the client is a legitimate MTA, not if it is a spambot. You can temporarily defer mail, accepting the message later if the sender tries again from the same IP address--a technique known as greylisting. You can even embed cryptographically secure expiration times in temporary mail addresses to validate mail before receiving the message body.

Compared to traditional spam filtering, filtering during the SMTP transaction also gives you more information. Mail Avenger collects a wide array of information about SMTP connections from clients, including TCP SYN fingerprints (which often identify the client OS) and network route information. Mail Avanger also flags properties of client SMTP implementations, such as whether they use pipelining, issue illegal SMTP commands, or deviate from the protocol in other small ways. Scripts can easily track this information on a per-sender basis using a simple database utility (included in the distribution). Thus, anomalies can be flagged when known senders exhibit radically different client behavior. Much of the information collected is also recorded in a new mail header, X-Avenger:, which can be fed to Bayesian content filters to improve accuracy.

A partial list of features:

Mail avenger is MTA-independent. It simply passes messages to a configurable sendmail program, and should therefore be compatible with any MTA that has a sendmail-like mail injection program. It has been tested with both sendmail and qmail, and others have reportedly used it with postfix.

Mail avenger is free software. It runs on Linux, OpenBSD, FreeBSD, and MacOS X, and will likely run with little or no modification on other Unix-like operating systems. Please let us know if you experience any portability problems.