pobox.com Microsoft Corp. Internet mail suffers from the fact that much unwanted mail is sent using spoofed addresses -- "spoofed" in this case means that the address is used without the permission of the domain owner. This document describes a family of tests by which SMTP servers can determine whether an e-mail address in a received message was used with the permission of the owner of the domain contained in that e-mail address. This memo defines an Experimental Protocol for the Internet community.