Hi @Simeon Steffen ,
i have an strange problem with Office 2016 Pro Plus on an domain joined terminalserver (Windows Server 2012r2).
Did this issue occur on any specific users/ clients or all users in your organization? And could those clients that not on domain joined terminal server work normally?
If it occurred on any specific clients only, please kindly check if there are any differences between this clients and others(such as client version, add-ins or other anti-virus programs)
According to your description, your Outlook client seems to be corrupted when you send / receive a message with your S/MIME certificate , in the point of Outlook, what's the version of your Outlook 2016(File>Office account>About Outlook), please first ensure that you have installed the latest update of Outlook.
Considering any third-party add-ins or anti-virus programs may also cause it, it's suggested to close other programs and test in Outlook safe mode to check if the issue has any difference(Windows + R > type "Outlook.exe /safe"> enter)
If the issue still exists, in order to confirm if it's related to your certificate itself, if it's possible, please try to switch to other certificate or use this certificate on other client for testing.
Also, if the issue occurred on those clients joined terminal server only, it may be more related to terminal server, it's recommended to post your issue in terminal-server related forum to get more targeted suggestions.
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