Microsoft today released a hotfix for the Windows 2003 SMTP stack that provides tarpitting for SMTP. (If you don't already know what tarpitting is, check this explanation). The idea is that you install software that intentionally slows down SMTP throughput for bogus requests. This helps make it uneconomical for spammers to ply their trade. The hotfix requires you to install a package and set a registry key, then you're done. Highly recommended.