I've always considered challenge / response spam. I typically turf any challenge's I get and give up trying to send that person an email.

While so-called challenge-response anti-spam systems have grown in popularity, they still seem like the wrong approach to spam. If you think about it, they basically assume all mail is spam, until someone proves otherwise. That sounds like they have a serious false-positive problem. All email is considered spam. For some, that's fine, but it can lead to other problems. Seriously overhyped challenge-response startup Mailblocks is discovering just some of those problems. It turns out that major ISPs, including AOL and Earthlink, are automatically treating Mailblocks' challenges as spam. In some ways, it's only fair. Mailblocks considers all mail spam until proven otherwise, so why shouldn't other ISPs consider their mail spam? Of course, what this means is that anyone who sends a mailblocks user an email from one of these ISPs doesn't know that his or her message hasn't been received and assumes it has been. Meanwhile, Mailblocks users have no idea that the person has emailed them either. Of course, what's most amusing about all of this is that Mailblocks is owned by AOL. Yes, that's right, AOL is blocking its own challenge-response emails as spam. (source: TechDirt)

You gotta love how intelligent those AOL folks are!!!