SMTP Service Extension for Trust of Reliable Origin
ETS
This document defines an SMTP service extension [ESMTP] whereby an SMTP client may establish a trust relationship with the server attesting its confidence in the validity of the originator of a message. This confidence, or its absence, can then be used to mitigate the cost of undesirable messages (primarily spam) without disrupting operations between, or with, SMTP implementations not supporting the extension.