Justin Mason's Anti-Challenge-Response Linkfest
SpamAssassin hero Justin Mason has posted a summary of opinions against challenge-response mail systems. I particularly like the pyschopathic challenge-response system user in the comments who defends blowback by arguing that it only affects a few innocent users.
Sorry, folks. CR fails my one question certification test for mail filter authors, and that's why it's not just bad and wrong, but profoundly antisocial--just like spam itself.