THIS IS A TEST INSTANCE. ALL YOUR CHANGES WILL BE LOST!!!!
...
Traditionally, the envelope-sender data has been added to messages as they pass through MTAs, but in varying ways (the Return-Path header, X-Envelope-From, etc.), and generally in terms of one header in the message. see EnvelopeSenderInHeaders.
A very helpful addition to support MUAs and MUA-level filters, would be if this was added to the Received header for the MTA handover, e.g.
...