That sounds like the Windows Mail viewer is being used rather than the Outlook one.
If you go to Preferences / Viewers / Plugins and configure the ActiveX + Preview + Office + Web plugin, there may be an alternative mail viewer in the list which you can assign the file extension you're trying to view to.
There are also some mail viewers written by other companies, as further alternatives:
Beyond that, the Windows Mail and Microsoft Outlook viewers aren't part of Opus or written by us; they're part of Windows and Outlook, and you're seeing the same error from them in File Explorer, so you may need to look to Microsoft for support for their viewers.