As all right-thinking people know by now, Challenge-response spam filtering is broken and abusive, since it simply shifts the work of filtering spam out of your email, onto innocent third-parties — either your legitimate correspondents, people on mailing lists you read, or even random people you have never heard of (due to spam blowback).
Anti Challenge Response Link Fest

What You Need to Know About Challenge - Response Spam Filters

Proper principles for Challenge/Response anti-spam systems