One technique that is very effective for blocking spam is to maintain one or more blacklists of spam sources, and then refuse all e-mail from those sources. The hard part is choosing the best blacklist(s), because different blacklists have different listing policies, and the more aggressive ones often list sources that send legitimate e-mail as well as spam. Other lists are so conservative that they block almost nothing at all. To aid in the selection of one or more of the public DNS-based blacklists I publish a comparison report that summarizes my weekly survey results.

I have started my own list of dialup zones, but it is not available as a DNS-based blacklist.

See how I have modified sendmail to block a large fraction of the spam that is sent to SDSC.

I hope to add more information as time permits.