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Content conversion: Invalid S/MIME encrypted message.
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I'm using Outlook 2016 with GpgOL 2.4.2 (Gpg4win 3.1.10) on Windows Server 2016 Standard (Terminal Server) and with Exchange 2013 on Windows Server 2012 Standard.

It's something similar as reported here

  • A local "user A" receives an external eMail from "sender X" (which is not part of this eMail server) and forwards it to another local "user B" without any modification.
  • The local "user B" replies to this eMail with some text and sends it back to the local "user A".
  • The local "user A" forwards this eMail with encryption back to the original external "sender X".

At this moment, the Exchange server replies immediately with:

Remote Server returned '550 5.6.0 M2MCVT.StorageError.Exception: ConversionFailedException, Content conversion: Invalid S/MIME encrypted message.; storage error in content conversion.'

There is NO attachment and S/MIME is NOT enabled.

The eMail in the Sent Items appears empty (everything is white, but in the eMail's source code there is some little markup).

I have found out that it happens also just when the local "user A" replies to the eMail of external "sender X", no matter whether the eMail had been forwarded or replied several times before or not. Just when replying to a new received unencrypted eMail with encryption enabled.
But it looks like this applies to the external "sender X" only.
I have tested it with my own received eMail (let's say "sender Y") and it worked.



Event Timeline

susedv updated the task description. (Show Details)
aheinecke added a subscriber: aheinecke.

This again,...
That error always occurs when the Exchange Server is unhappy with the structure of our PGP/MIME Mails. It has nothing to do with S/MIME, that is only because Exchange only knows about S/MIME, so our PGP/MIME Mails also claim to be S/MIME mails.

We need to find out more why this happens with mails from Sender X and not from Sender Y. Because I just tried it and I cannot reproduce it. Currently I have no Idea what the Sender could have to do with our resulting MIME structure.

I'm not sure if a sample mail would help or if this is related to the server.

One way where I can imagine this happening would be if UserA has an Exchange 2007. We don't fully support Exchange 2007. Any chance that you could find that out?

I have checked the eMail header of the eMail from Sender X in the Exchange mailbox of User A and I see Sender X is using Mozilla Thunderbird and I tested it with Thunderbird also, but it works for me.
I cannot provide all details of the eMail from Sender X because it's a customer of another customer, but I have replaced the IP addresses and other private information in the eMail header and this is the result:

Received: from hostname-of-exchange-server-of-user-a-and-b.domain.local ( by
 hostname-of-exchange-server-of-user-a-and-b.domain.local ( with Microsoft SMTP Server
 (TLS) id 15.0.1473.3 via Mailbox Transport; Fri, 10 Jan 2020 10:50:01 +0100
Received: from hostname-of-exchange-server-of-user-a-and-b.domain.local ( by
 hostname-of-exchange-server-of-user-a-and-b.domain.local ( with Microsoft SMTP Server
 (TLS) id 15.0.1473.3; Fri, 10 Jan 2020 10:49:43 +0100
Received: from ( by
 hostname-of-exchange-server-of-user-a-and-b.domain.local ( with Microsoft SMTP Server
 id 15.0.1473.3 via Frontend Transport; Fri, 10 Jan 2020 10:49:42 +0100
Return-Path: <>
Received: from localhost (localhost [])
	by (Postfix) with ESMTP id CED701827F8
	for <>; Fri, 10 Jan 2020 10:44:49 +0100 (CET)
X-Virus-Scanned: Debian amavisd-new at
Authentication-Results: (amavisd-new);
	dkim=pass (2048-bit key)
Received: from ([])
	by localhost ( []) (amavisd-new, port 10024)
	with ESMTP id EMaXWGoQIPdQ for <>;
	Fri, 10 Jan 2020 10:44:49 +0100 (CET)
Received-SPF: Pass (sender SPF authorized) identity=mailfrom; client-ip=;;; 
Received: from ( [])
	by (Postfix) with ESMTPS id 7321818224A
	for <>; Fri, 10 Jan 2020 10:44:49 +0100 (CET)
Received: by with SMTP id g17so1158467wro.2
        for <>; Fri, 10 Jan 2020 01:44:49 -0800 (PST)
DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed;; s=20150623;
X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed;; s=20161025;
X-Google-Smtp-Source: APXvYqyWcGI+/wov0T_TRUNCATED_FOR_PRIVACY
X-Received: by 2002:a5d:67c7:: with SMTP id n7mr2450113wrw.319.1578649488292;
        Fri, 10 Jan 2020 01:44:48 -0800 (PST)
Received: from SenderXs-MacBook-Pro-2063.local ( [])
        by with ESMTPSA id f17sm1616273wmc.8.2020.
        for <>
        (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128);
        Fri, 10 Jan 2020 01:44:47 -0800 (PST)
To: =?UTF-8?Q?User_A=c3=b6?= <>
From: Sender X <>
Subject: =?UTF-8?Q?Subject_B=c3=bcro?=
Autocrypt:; keydata=
Message-ID: <>
Date: Fri, 10 Jan 2020 10:44:46 +0100
User-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.15; rv:68.0)
 Gecko/20100101 Thunderbird/68.3.1
MIME-Version: 1.0
Content-Type: multipart/alternative;
Content-Language: de-DE
X-Proxy-Ident: 0/15769-1-1578649629
X-CTCH-RefID: str=0001.0A0C020F.5E18481E.003D,ss=1,re=0.000,recu=0.000,reip=0.000,cl=1,cld=1,fgs=0
X-MS-Exchange-Organization-Network-Message-Id: f5d63cec-8127-4d5b-d786-08d795b26b47
X-ESET-AS: R=OK;S=0;OP=CALC;TIME=1578649801;VERSION=7844;MFE-VER=58;MC=2128443591;TRN=21;CRV=0;IPC=
X-ESET-Antispam: OK
X-EsetResult: clean, is OK
X-EsetId: 37303A29C7AE4F63657067
X-MS-Exchange-Organization-AVStamp-Enterprise: 1.0
X-MS-Exchange-Organization-AuthSource: hostname-of-exchange-server-of-user-a-and-b.domain.local
X-MS-Exchange-Organization-AuthAs: Anonymous