1. A computer program product, comprising: a computer storage medium and a computer program code mechanism embedded in the computer storage medium for causing a computer to process electronic mail messages, the computer program code mechanism comprising: a first computer code device configured to receive an incoming electronic mail message; a second computer code device configured to determine a candidate machine and a candidate user id of a purported sender of the incoming electronic mail message; a third computer code device configured to send a verification request to the candidate user id at the candidate machine; a fourth computer code device configured to receive a verification response to the verification request; and a fifth computer code device configured to block delivery of the incoming electronic mail message based on the verification response when the response indicates that the candidate machine does not exist.