At the moment I am using i3 and neomutt and trying to sort out smtp servers with multiple accounts and looking at an email header I see
Received: from relay11.mail.gandi.net (relay11.mail.gandi.net [217.70.178.231]) by w4.tutanota.de (Postfix) with ESMTPS id DCBE610602E3 for <i###@tutanota.com>; Sun, 14 Feb 2021 14:45:08 +0000 (UTC) Received: from newdev.siduction.org (unknown [########]) (Authenticated sender: ###@#####t.uk) by relay11.mail.gandi.net (Postfix) with ESMTPSA id 58401100005 for <i###@tutanota.com>; Sun, 14 Feb 2021 14:45:08 +0000 (UTC)
what is "newdev.siduction.org" I might expect to see a reference to Linux or Neomutt but not siduction.