Before allowing e-mails through to your in-box, Mailblocks automatically transmits a numerical password to first-time correspondents. The senders must then retype the code into an onscreen dialog box before the system acknowledges them as legitimate. Mailblocks also has reverse-engineered a way to fetch a user's messages from other Web e-mail systems and similarly screen out unwanted cyberjunk.

http://www.mailblocks.com/