Stata Labs' SAProxy

[http://www.statalabs.com/products/saproxy/]

There used to be a free version. Their FAQ says:

What happened to the free version? - Last year, Stata Labs offered a free open source version of SAproxy 1.2. This version could also be found in the Open Source community for SpamAssassin. However, the product did not have a simple configuration or intuitive user interface, and, as a result, was difficult for the majority of users to understand and implement. In order to bring the power of SAproxy to a larger audience of users, Stata Labs put in a significant amount of effort to create, SAproxy Pro, which is easier to use, configure and refine. SAproxy Pro is the commercial version of SAproxy, complete with support and ongoing improvements.