Integrating Spam Assassin with your Mail Transfer Agent.

I would first take a look at IntegratedInMta (I believe this will address the below TODO for postfix; at least it did for me)

Here are links to documents on how to integrate SA with Mail Transfer Agent

[http://www.exim.org Exim] - [http://www.exim.org/exim-html-4.50/doc/html/spec_40.html Content Scanning] with Spamassassin Documentation Page.
[http://www.stalker.com CommuniGate Pro (CGP)] - Try CGPSA, available at http://www.tffenterprises.com/cgpsa/
[http://www.stalker.com CommuniGate Pro (CGP)] - CGPAV can do SA and ClamAV, available at http://program.farit.ru/
[http://www.postfix.org Postfix] First Step - Basic Integrating - at http://onetforum.com/fourm/viewtopic.php?p=27
[http://www.postfix.org Postfix] Secound Step - Basic Integrating with SPAM Quarantine - at http://onetforum.com/fourm/viewtopic.php?p=63#63

[http://www.gurulabs.com/training/GL275-Enterprise_Linux_Network_Services.php Guru Labs course GL275] - Feaures in-depth Spamassassin coverage (lecture and lab exercises) including MTA integration with Sendmail using milter and Postfix integration both after and before queueing