Review of MailBlocks Service can be found at poochkiss.com. I signed up yesterday and so far so good!

"The Mailblocks antispam service is based on a so-called challenge-response mechanism to block bulk mail sent automatically to e-mail accounts. When a customer receives a new message from an unknown correspondent, the system will intercept the message and automatically return to the sender a digital image of a seven-digit number and a form to fill out. Once a human being views that number and types it into the form - demonstrating that he or she is a person and not an automated mass-mailing machine - the system will forward the e-mail to the intended recipient."