Abstract: A computer system and method for filtering unauthorized electronic mail messages that are sent by senders to a user. The firewall intercepts any incoming email sent to the user and checks it against a master list of allowed senders. Any sender not found on the list is sent a user-configurable challenge that must be responded to within a set time limit. If the challenge is not successfully met, the sender can be blocked and the mail can be bounced, discarded, or sent a removal request.