milter manager is a flexible and low administrative cost anti-spam system that can be used with MTAs that support milters like Sendmail and Postfix. It provides an effective solution by combining a range of existing milters.
OpenDKIM is a DKIM implementation with both a C library for writing your own applications and a milter-aware filter for use with recent MTAs.
A network access control (NAC) system.
A library that implements Perl 5-style regular expressions.