I'm sure challenge-response works, but my problem isn't *nearly* bad enough to make me want to hassle my friends, family, aquaintances, and business contacts witht that.
I preloaded people like that in my address book so they've never received any challenge. I only use this system on two addresses which get heavily spammed. The others get the regular filter.
I'm dubious about the challenge/response system for anything other than personal users. I get far too many auto-generated e-mails that I need to see ...
They all pretty much come out of the same domain names right? My ISP's filter takes domains as well as individual addresses. None of the email is ever lost. It goes into a spam trap that you can review and then delete. I choose not to review it. Once you set it up and monitor it for a short time to make sure no one who should be reaching you falls in to it, there's really no need to keep monitoring. I've spot checked it periodically and it's always pure spam. Humans who need to get thru after that have always responded.