Personally, I find challenge-response emails particularly irritating. I ran Spamassassin and then if the score was 2+, it would run TMDA which would do a human verification auth request. I got almost no spam. And I found that people found it confusing to reply or hit a link for mail to be delivered, so, I was missing those messages too. Mailing lists and autoreply boxes were problematic at best.
TMDA could also do things like have email addresses that were temporary, or assigned to a particular email address, etc. So, I could set up an account like
mcduw.a09d93fa@domain.com (it would generate it) and it would only accept mail from
uselesswarrior@domain.com, and I could choose whether it dropped or requested verification if someone else used it.
Spamgourmet is another one. You could set up an address like
mcd.4@domain.com, and it would allow 4 emails to come through to that box, and trash anything after that.
I can see why people would use it -- submitting to many TGPs/linklists is liable to get your email handed off to dozens of spam factories, however, its a delicate mix.
In reading this, I realize I said almost nothing in relation to your question.
I guess, if you don't like it, delete them. Maybe look at the submissions to see if it is worth it. You could make it a new rule.
