Tarpitting

Initially, spamd was only a tarpit. (Because DNS RBL's suck wet hairy moose rocks)

Configure blacklists from sources obtainable on the internet

Redirect blacklisted hosts in pf to spamd

spamd talks to them very very slowly. (1 byte per second, TCP Window size 1)

About as effective as DNS rbl's, but made spammers hurt and wasn't subject to DNS poisoning of RBL sites.