jASEN is a pure java Anti Spam ENgine combining bayesian-like scanning with intelligent email inspection and classification. jASEN is best suited to developers wishing to integrate anti-spam services into an existing server based java email application.
Cheescloth is a security script that is used to monitor and block email being brute force sent to your mail server by zombie machines in control by spammers and hackers. It also will catch and block hackers trying to hack your server via SSH connection
iXhash is a plugin for the popular spam filter SpamAssassin. It computes MD5 checksums of fragments of the body of an e-mail and compares them to those of know spam using DNS queries to a RBL-like name server. In effect this gives SpamAssassin access
A user-centric Java Mailet for the Apache James mail server (a Java mail server) for the purpose of filtering spam using a naive Bayesian algorithm. As a Mailet, the filter can act upon spam as it is received over SMTP before the user gets it.
qmail-monitor is a patch to qmail that offers mail monitoring (duplication) functionality on both incoming and outgoing smtp traffic. the monitor ruleset is intuitive and allows the administrator to make changes w/o restarting the server processes.