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
eWall - a programmable SMTP proxy with built-in SpamAssassin and ClamAV clients, an open, PHP-driven API, an extensible open source Repository, currently supporting all major SPAM filtering techniques (SPF, DNSBL, SURBL, harvester detection etc.), message Quarantine, SSL support and a Remote Admin Panel. View the full feature list. Free version available!
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.
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