Hallo Uwe,
ich bin mir nicht sicher, ob ich Dich richtig verstanden habe. Das Mailrelay nutzen die Clients selbst überhaupt nicht. Nur der Mailbenutzer des Groupware-Servers darf hier einliefern.
Die header Zeilen einer solchen Mail:
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for <anderer.benutzer at unsere-domain.de>; Tue, 29 Jan 2019 09:09:45 +0000 (UTC)
Date: Tue, 29 Jan 2019 09:09:44 +0000
To: anderer.benutzer at unsere-domain.de
From: "Prof. Dr. Ein Benutzer" <ein.benutzer at unsere-domain.de>
Reply-To: Ein Benutzer <ein.benutzer at persoenllich.com>
Subject: Schnelle Bestellung #*2902:2019*#
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Mich beunruhigt auch etwas der gefälschte Return-Path. Oder lässt sich der so ohne weiteres ändern?
Viele Grüße
Sebastian