Enhancements to Postfix for SPAM filtering
Greylisting has been implemented on a IP address level. Basically, your sending machine is marked as "grey" for about 10 minutes. If you resend your message after 10 minutes, the machine will be marked ok. If you want the ugly details, or your a small business that wants to do this, a tarball is here with the changes we've made to postfix.
Update in 2007: We no longer accept mail from unknown or dynamically allocated dhcp addresses. Sorry. The spammers have started to resend to overcome greylisting.
Update May 2008: In the ever escalating war with spam, we now also check the registration name server. Certain ones, known to us to enable spammers have been banned as well. domaincontrol.com are you listening?