Had to look into a mail denial of service at work today. I was extremely surprised that a company the size of ours had never really dealt with an email DoS before. Unfortunately the guy was using open relays, and the DNS blacklists we offer to our clients didn't have the relays he was using listed. Bah. Spamcop did though, as did osirusoft and dorkslayers, so it looks like we're going to have to expand our blacklist usage somewhat.
People who start these things are really sad little idiots. It makes me wonder how Slashdot (and thus use.perl) deals with their DoS attacks, because I've rarely seen use.perl slow down much.