I've been giving more thought to greylisting, a process for temporarily delaying emails sent to my server, to help cut down on the spam that ultimately makes it through to my server. Some of my filters are being a bit problematic, and I've had a very small number of legitimate mails that didn't make it through, so I'm needing to revisit things again.
I've found an interesting piece of code called Gld, it uses MySQL so it should be fairly efficient, but I'm having trouble compiling the darned thing for OS X. Unfortunately, it's poorly documented and I'm waiting for the author to respond to my plea for help. But, it sounds promising, if I can make it work here.