A method and system for the policy-based restriction of electronic mail transmissions. A method for classifying electronic mail message transfer requests for policy enforcement can include identifying a source of an incoming electronic message, classifying the source, and applying a message transfer policy associated with the classification for the source. In particular, the identifying step can include identifying a network address for the source. The classifying step by comparison, can include classifying the source as one of a trusted source, a blocked source, and a suspect source. The classifying step also can include classifying the source as one of an authenticated source and an anonymous source. Finally, the classifying step further can include classifying the source as a blocked source where the source appears in a realtime black hole list.