Spam/AntiSpam

• Blitzed Open Proxy Monitor List
• Mail Relay Tester
• Open Relay Database
• Spamhaus

• Spam Proxy Daemon
• TarProxy
• SA-Exim
• Anti-Spam SMTP Proxy
• Confirm - a nice procmail add-on

HOWTOs

• SpamAssassin on Mac OS X HOWTO