Are Challenge-Response Users Missing 20% of Their Email?

Challenge-response systems work to minimize junk messages by validating that each message is really from a human being.

Most such spam prevention schemes confront all would-be email senders with a CAPTCHA test; requiring them to correctly identify a jumbled word or image. Users that successfully respond are allowed to have their email pass through unfettered.

The main concern regarding this approach is that many users simply aren't going to bother responding to a challenge. My own personal response when I receive such messages is one of annoyance. I just about always play the game of responding to the CAPTCHA, but if I was already looking for a reason not to write back, I might "forget" to do it.

TotalBlock is an Australia-based anti-spam company that uses challenge-response technology. TotalBlock's own survey indicates that only about 80% of people who get challenged by such systems go on to correctly reply to the challenge. While user's can manually "whitelist" email addresses, or retrieve messages for which the user failed to respond to the CAPTCHA - both of these scenarios require the recipient take additional steps.

Also in question is the premise that all machine-generated traffic isn't wanted. This seems like a poor assumption. Purchase receipts, auction notifications and email bounce messages are all examples of machine-generated email that end users will almost always want to get. In all fairness, many anti-spam filter programs have problems with machine-generated email whether using challenge-response or some other technology.

Most amusing is the possibility that a challenge-response system might cause havoc when attempting to exchange messages with other similar systems. Consider the following possible scenario:

Most systems have measures in place to prevent the last two steps from repeating out to infinity. Further - some (not all) systems are smart enough not to challenge users which have been sent email previously. Still, it seems quite reasonable that many users of challenge-response systems won't be able to communicate with each other they take the time to manually add their addresses to their challenge system.

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