A few weeks ago, somebody on the Mailop list asked about an email they received. Turns out, it was a Challenge/Response spam filter. You know, one of those emails from a robot that says "click here to prove you're not a robot, otherwise I'm not going to deliver your email on to the intended recipient." That doesn't work very well -- do you think Amazon has people waiting to click on that link each time they send out an emailed order receipt? Yeah, yeah, supposedly you (the user with the C/R spam filter) can whitelist that stuff, but you're going to miss something.
Challenge/Response filtering seems to have mostly disappeared; this was the first time I had heard about it in many years. But it's not the first time I've blogged about it; look all the way back to this MediaPost article from 2006, and you can see me raise still-extant concerns relating to how C/R works.
Click here to jump in the time machine and read what else I've written about C/R in the past.