I lurk on a mailing list about e-mail lists, and recently the list has been going back and forth over using challenge/response systems to control SPAM.
In a challenge/response system, typically if you e-mail an individual using such a system, you will get back an automated message which requires you to reply to it in order to prove you are not a spammer. Some systems require complex “challenges” to demonstrate that you are not a spammer.
Personally, I consider all challenge/response e-mails to be just as annoying as spam and tag them as such within my e-mail client. Such e-mails are especially annoying since 99 percent of the the one’s I’ve received have come from people who contacted me first. If someone’s going to e-mail me requesting a response and then not take the time to add me to their white list, it must not really have been as urgent as they claimed in the e-mail.