Spam Prevention Links
This online article by Lorrie Faith Cranor and Brian A. LaMacchia discusses issues and trade-offs in spam and spam prevention.
Mail Abuse Prevention System
Includes Realtime Blackhole List (RBL) and Relay Spam Stopper (RSS).
Open Relay Behavior-modification System (ORBS)
System lists all known open relays.
Fight Spam on the Internet
Collection of antispam links and resources.
MAILSweeper, Content Technologies
Filtering technology allows sites to sort email content through various text rules. Popular engine for filtering spam and viruses.
CommandView Message Inspector, Elron Software
Content filtering system for inbound mail is also capable of filtering outbound email to avoid legal liability or security breach.
I list this link to give you an idea how low spam practitioners will go. You can scan Web pages for new email addresses, forge email headers, and send email from ISP shell accounts. Previously listed systems can block most of these efforts.