1. A method of classifying e-mails, the method comprising: receiving e-mails that include a header portion and a content portion; storing the received e-mails; analyzing the content portions of the stored e-mails to cluster the e-mails into multiple groups of substantially duplicate e-mails; storing the multiple groups of substantially duplicate e-mails; selecting one or more test e-mails from at least one of the stored groups; determining a class for the one or more test e-mails; and classifying at least one non-test e-mail in the at least one stored group based on the determined class of the one or more test e-mails.