Check all incoming E-mail sent from people who know or not. The source and destination of the information, E-mail has been the topic clear contacts (if you're not sure, suspect them first).
Check E-mail sent to every person that comes from a known source and destination information, E-mail, must be subject to a definite contact. (If unsure, suspicious).
Check all the E-mail sent came from known individuals or data source, and the destination of the E-mail valid topics contact clear. (if not to suspect).