Just finished writing the documentation on adding greylisting as a new anti spam feature to our mailserver setup. Grey listing is a combination of whitelisting and blacklisting senders and mailservers. It does this all automatically, basically it asks all non whitelisted senders to try to resend the message after 5 minutes. Every proper MTA (mailserver) will do this without a problem. If the sender and mailserver have done this a number of times they will be whitelisted as a sender that does not send spam. (If you want all the details on greylisting read the site greylisting.org)
I’ve got it running for a few days now and my spam has almost dropped to zero. Normally my Junk folder would at least have 20 spam messages a day which are for 99% caught by DSpam and Mail.app. Meaning that this really helps, it will even improve the DSpam filtering capabilities as the spam/ham ratio is better now.
There is however a drawback to this setup. All email which is send to you from an irregular source will be delayed for a minimum of 5 minutes. If that is not a problem feel free to add this feature to your mailserver.
Also note that if you use a backup MX this backup server should also have the same greylisting setup otherwise all mail from your backup server will bypass the greylisting as your backup server will be whitelisted. If you don’t have a backup MX server with these capabilities please contact me as I think I might be able to help you out.