In the final RFC 4408 specification, two elements of the message are used to determine the domain considered as the sender: the domain specified by the SMTP “HELO” (or “EHLO”) command and the domain specified by the address of the “Return-Path” (or “MAIL FROM”) header, which is also the bounce address. Different considerations make it possible to take into account one of these values only; we recommend making sure that both sources specify the same domain.