It seems that Challenge-Response solutions to the spam problem are getting more popular, with lots of people using systems like ASK, TMDA and several commercial systems (including Earthlink now offering C/R to their home users).
Now I hate C/R systems. With a passion. I absolutely will not respond to them. They go in the trash. I don't get them very often but I get them more and more. I think they have the potential to seriously damage email communication as we know it. And I'm not alone in this opinion.
I think the above is wrong though. We're all familiar with C/R systems in mailing lists - you get a challenge when you subscribe to confirm that your email address is valid and you really wanted to subscribe. That confirms that you want to receive mails from the list. But with a C/R anti-spam system you're confirming that you want to send mails to somewhere, and that seems backwards to me. If I didn't want to send something there I wouldn't have, umm, sent something there, and I'd really rather not be questioned about it, thanks.
As a company we need to be able to deal appropriately with the rise in C/R systems, and be able to talk about them sensibly if we're going to dismiss them as a solution, so feel free to post any pros/cons to this entry. I have a list of my own somewhere but I'm interested in other people's opinions.