Return-Path header
The header the delivering mail server adds at final delivery to record the envelope sender, the address bounces for the message go to.
What it means
Return-Path is a trace field written at final delivery from the MAIL FROM address of the last SMTP transaction. RFC 5321 says an originating system should not send a message that already has one, and a delivering server may remove existing ones before adding its own. For bounces and other notifications its value is empty: Return-Path: <>.
In a delivered message, Return-Path is the easiest place to see the envelope sender, and therefore the domain SPF checked. It is often a long address at the sending platform or at a bounce subdomain of the sender, sometimes encoding the recipient (VERP). RFC 3834 tells automatic responders to reply to the Return-Path address rather than to From.
Example
Return-Path: <bounces+ana=example.net@mail.example.com>Related terms
Sources
What this definition rests on. The wording is ours; the facts are theirs.
All 432 terms in the glossaryThe wiki: blocklists and bounce codes