Anti-spam mailserver stratagies
Greylisting: "Greylisting relies on the fact that most spam sources do not behave in the same way as 'normal' mail systems."
Postgrey - Postfix Greylisting Policy Server
DSPAM: "The DSPAM (as in De-Spam) Project is a scalable, open-source statistical anti-spam filter"
Spamhaus: You can purchase a data feed of IPs that are sending out spam - the "black hole" list.
Detailed Comparison of Bayesian Spam Filters: The author conducts an empirical, quantitative study of the out-of-box performance of Bogofilter, CRM114, DSPAM, SpamAssassin, SpamBayes, and SpamProbe. SpamAssassin (a perl script) comes out most accurate but slowest, at 1.1 seconds per message. Bogofilter (a C program) clocks in at 0.02 s/m! Be sure to read the contributed comments as well for some very useful information.
Postfix Anti-UCE Cheat Sheet: Detailed postfix configuration instructions for spam fighting.