Eliminating spam was the first request of prospective users surveyed by the company last fall. Mailblocks tackles this problem by placing every e-mail not from someone in the recipient's address list in a pending folder. The system then ships the sender a message with a random number presented in a specially color-coded manner that is not machine readable. When the sender reads and types in that number the system sends a signal to move the e-mail to the recipient's inbox.The system itself will undoubtedly create extra overhead for Mailblocks' systems, but maybe not for too long. If the service becomes popular enough, spammers may end up filtering @mailblocks addresses out of their bulk mailing lists.
Honestly, I can see a few instances where the requirement of a secondary message could inconvenience senders of legitimate email, but it seems like a small price to pay. Unlike other "whitelist" email filtering services, Mailblocks at least automatically generates a message asking for a confirmation from an unknown sender.
At only $10/year for 12MB of storage, Mailblocks seems like a pretty good deal, especially with a current promotion that throws in an extra two years of service for free.