Accurate, multi-platform spam filter updated several times per day using 60, 000+ heuristics. Typically scans messages in under 200 milliseconds on modest hardware. [Unix, Windows]

The Active SMTP Appliance is a hardware appliance that dynamically filters and quarantines junk e-mail before it gets to the mail server. [Hardware]

Mail Warden Pro is E-mail server security software with anti-spam, anti-virus and content filtering for any mailserver. [Windows]

A Java program using a Chi Square algorithm to detect spam. It is best suited to developers wishing to integrate anti-spam services into an existing server based java E-mail application, but can be used for client applications. [Java]

A small, fast, open-source spam filter. Designed to work with or replace SpamAssassin. Uses DNSBL lookups to discriminate spam. [Unix]

Tethernet TetherFilter is an easy to deploy, affordable anti-spam and anti-virus appliance that can be used in front of any existing Internet email server. [Hardware]

Spam killer software. It blocks spam before it is even delivered to your mail server. [Windows]

Free open-source software for users of the free Windows Hamster local news and mail server, to filter out spam using any of the publicly available Realtime Block Lists (RBL). [Windows]

This article explains how to set up ACLs in Exim 4 to reject spam.

Software intercepts spam, email-borne viruses, and other objectionable content before reaching your organization's mail server by utilizing complex contextual signatures as opposed to simple keyword lists. [Unix, Windows]