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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.

Actually, the Return-Path is standard now,
and I don't see any need to add yet another
way to specify the envelope recipient. Go
read RFC-2821 section 4.4 where this header
is defined as a MUST.

-- Arik Baratz

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.

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