Integrating Spam Assassin with your Mail Transfer Agent.

I would first take a look at IntegratedInMta (Sendmail, Postfix, Qmail)

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.nospamtoday.com/server/ NoSpamToday! SMTP Proxy] Seamless SpamAssassin Integration Into Any Existing E-Mail Environment - No Spam Today! requires just a few mouse-clicks to integrate SpamAssassin into any existing Email infrastructure (supports Windows NT/2000/XP/2003 and Linux) - at http://www.nospamtoday.com/server/
[http://sssolutions.net/ew/ eWall] - is a programmable SMTP proxy for any mail server, allowing to create flexible filtering scenarios, using set of pre-defined conditions/actions and [JavaScript] engine. Among other features, eWall supports antivirus software (ClamAV, AVG, Avast!, CA eTrust, etc. ) and of course, can be integrated with SpamAssassin. There's a special video guide on [http://sssolutions.net/ew/tutor.php?topic=sa this page].

[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