I activated BoxTrapper on my personal email account, which utilizes "challenge-response verification" similar to that of Spam Arrest.
BoxTrapper is bundled as part of CPanel, but not sure if exclusive to it. It definitely works, as nearly all of my spam has been eliminated. Cpanel's Boxtrapper documentation is located at
http://twiki.cpanel.net/twiki/bin/vi...ocs/BoxTrapper