The weakness of challenge/response anti-spam measures -- a system where suspect email is diverted into a spam bin and the sender of the message is tasked with verifying his or her identity -- was made clear to me this morning. It will never work because of lazy sods like me. Allow me to offer this illustration.
I'm one of those people who block their phone number from being displayed on Caller ID systems. No, I'm not afraid of waking up one morning to see a swarm of black helicopters over my home. This is just one of those things that fall under the "none of your business" heading.
The drummer in my band installed a service on his phone line that requires callers to reveal their phone numbers by preceding his number with *82. The result? I contact him only through email.
Should he adopt a challenge/response scheme for that email, he won't hear from me at all. And that's the fatal flaw of challenge/response. There are too many ornery cusses like me.
For example, just yesterday a reader of this very blog sent a message responding to something I'd written. I replied to his message and within a minute or so I received an automated response demanding that I click a link to prove I wasn't a spammer. I'm sorry, you're asking the guy who can't be bothered to punch an extra three buttons on his phone to click a link, wait for a webpage to load, and fill out a request form in order to send email to a perfect stranger?
You see, we of the Ornery Cuss class resent the fact that someone else makes their email problems ours. We've carefully crafted our spam filters to weed out the majority of this effluvia, and it irks us the tiniest bit that others can't be bothered to do the same. Yes, you'll get less spam, but my guess is that you'll get less legitimate email as well.
And god help you if you've also asked your friends to punch *82.