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

CleanMail SMTP/POP3 Proxy - Seamless integration of SpamAssassin and ClamAV into any Windows or Linux-based email processing environment with just a few mouse-clicks - see http://www.byteplant.com/products/cleanmail/ for details.
CommuniGate Pro (CGP) - Try CGPSA, available at http://www.tffenterprises.com/cgpsa/
CommuniGate Pro (CGP) - CGPAV can do SA and ClamAV, available at http://program.farit.ru/
Exim - Content Scanning with Spamassassin Documentation Page.
Postfix First Step - Basic Integrating - at http://onetforum.com/fourm/viewtopic.php?p=27
Postfix Secound Step - Basic Integrating with SPAM Quarantine - at http://onetforum.com/fourm/viewtopic.php?p=63#63
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 this page.

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