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.