http://www.boston.com/business/technology/articles/2009/04/11/filters_getting_better_at_blocking_spam/?page=2

Other companies, like Sendio Inc. in Irvine, Calif., and Spam Arrest LLC of Seattle, use a “challenge-response” technique. Send an e-mail to a challenge-response user and you’ll get an automated reply, asking you to type in some words or numbers. This will prove your e-mail came from a human being and not a spam-spewing computer. If you send the correct reply, all your future messages are delivered immediately, but spam messages can’t get through.

For the record… Sendio’s sender address verification technology (SAV), also know generically as challenge response, DOES NOT require anyone to “…type in some words or numbers.” Our technology requires a simple “REPLY & SEND.” and ONLY in the case where the sender is completely unknown to the intended recipient. For example, anyone I send an e-mail to is automatically added to my personal accept-list, thus, is NEVER subjected to the address verification process.