Email Question: Challenge-Response Email filtering

Not necessary to send them to a web page - most challenge and response systems simply require them to Reply to the challenge -- which, because spambots almost never use real return addresses, the spammers will never do.

I think most of these services are hosted externally, not filters thet run on the Mac. Your ISP may have such an option on your basic POP account.

A warning about the downside: It is really offputting to some people like me, for someone to email me asking for advice, then make me jump through hoops and take more time just to get the reply through - it's penalizing me for the sins of others. At some point, there will be a backlash. In my case, for example, I may delay responding to the challenge until I'm not so busy, or not respond at all.