Spam Blocking with a Dynamically Updated Firewall Ruleset

In this paper, we detail our methods for controlling spam at a small ISP, reducing both resource usage and customer complaints. We will discuss our initial unsuccessful tactics, and the resulting development of our unique spam blocking system. Deny-Spammers classifies hosts as probable spammers and inserts those hosts into a dynamically updated firewall ruleset on our mail server, thereby effectively blocking the host from making an SMTP connection to our mail server. Our analysis demonstrates that this has been effective in reducing the amount of spam that our customers receive, and the burden on our limited resources.