The Anti-Spam SMTP Proxy (ASSP) server project is an Open Source, Perl based, platform-independent transparent SMTP proxy server available at SourceForge.net that leverages numerous methodologies and technologies to both rigidly and adaptively identify e-mail spam. ASSP is easy to set up because it requires only minor changes to the configuration of your Mail Transfer Agent.
From a technical point of view, the software is written in an obfuscated style which makes it difficult to debug, and update. For example, the entire program is written as a single Perl program file, rather than being broken into modules.