1. An e-mail server that facilitates detection of unsolicited e-mail, comprising: a processor configured to execute: an e-mail component that receives and stores messages and computes associated probabilities that the e-mail messages are spam for valid but unmonitored e-mail addresses; a mail classifier that receives e-mail messages and determines the associated probability that a received e-mail message is spam for each of the messages; and a challenge component that sends a challenge to an originator of the received e-mail message to a valid but unmonitored e-mail address, where the received e-mail message has an associated probability of being spam greater than a first threshold and a difficulty of the challenge being based, at least in part, upon the associated probability that the e-mail message is spam, and the challenge component desists from sending the challenge to the originator of the received e-mail message to the valid but unmonitored e-mail address, where the associated probability of the received message being spam is less than or equal to the first threshold, the challenge being sent, at least in part, to obfuscate validity of the valid but unmonitored e-mail address to the originator of the received e-mail message.