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 https://dev.gnupg.org/T4321.
- 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.