1. A method for filtering electronic mail (e-mail) sent to a user e-mail system comprising: receiving at the user e-mail system a current e-mail message sent from a sender address, wherein the e-mail system has a plurality of different levels of security ranging from high to low, each level of security associated with a different set of one or more e-mail filters; determining that a current level of security has been selected by the user, wherein the current level of security as selected by the user is associated with a challenge-response filter; determining that the current e-mail message is not a challenge e-mail response message sent from the sender address in response to a previous challenge rendered by the user e-mail system, the previous challenge having been identified at a computing device associated with the sender address by a valid challenge key in the previous challenge rendered by the user e-mail system, the valid challenge key identifying a message as a challenge requiring a challenge e-mail response message; returning to the sender address a challenge that requires a correct challenge response, wherein the challenge rendered by the user's e-mail system is selected from a plurality of challenges generated by an external provider of challenges, the challenge being sent to the sender address via e-mail; storing the current e-mail message sent from the sender address in an intermediate buffer coupled to the user e-mail system; receiving a challenge e-mail response message sent from the sender address in response to the challenge rendered by the user's e-mail system; and evaluating the challenge e-mail response message to determine whether the challenge response message includes a correct challenge response and corresponds to a formal criterion, wherein the formal criterion is a server of the sender being configured to accept e-mail, wherein future e-mail messages from senders associated with servers that do not accept e-mail are prevented from reaching an intended recipient at the user e-mail system.