send non-delivery notification
This page suggests that the feature exists in Mail under Lion. The feature, however, does not appear to exist, either as a toolbar option, nor as a Message menu command, nor as a context menu command, nor as an item in the Help file for Mail. The aforelinked document says "Lion" in its title, but refers to Mail 4.0; the Mail program under Lion is Mail 5.1.



This may or may not have anything to do with Apple's acknowledgement that bounce does not always dissuade spammers.