If you run your own email server, there is no avoiding spam. Spam comprises between 75 and 99 percent of all email coming into an email server. On my server I was getting around 7k spam messages a day. I will be posting a few tips for handling spam in the coming days, but below is a new method that I'm told is quite effective. It's called "grey listing." As the term implies, it is somewhere between black listing and white listing.
Grey listing works based on the fact that most spammers are so busy spewing vast numbers of email messages that they will go around any address that does not respond quickly. So, based on that knowledge, the email server's grey listing program tells every new sender that the account to which they are sending is busy and to try again later. This error message is a legitimate email server message. A legitimate email server will try again within an hour. The grey listing system will accept the email message on the second try and will put the sender into the approved list so that no further delays will take place. Spammers will usually not try again. Of course, as this new trick is employed more widely, we have to assume that spammers will adjust.
Give it a try and let me know how it worked for you.
Everything Sysadmin: Anti-spam trick: Grey listing