Getting this message - Outlook 2016 Error mesage: 'Either there is no default mail client or the current mail client cannot fulfil the messaging request. Please run outlook and set it as the default mail client'. The default mail client is Outlook 2016. This is happening after I have copied and pasted emails into Directory Opus File Explorer - it also happens on Windows File Explorer. Any suggestions on how I can view the email and stop the error message? Thanks.
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.