Recently when user upgraded to a Windows 10 PC and had O365 Business account (so also a desktop version) to fix an issue with Opera3 emailing we had to carry out the following: 

1. I uninstalled the Opera3 client and deleted all folders   
    c:\programfiles x86\pegasus
    c:\programdata\pegasus
    c:\programfiles x86\common files\pegasus 
    I also removed from the registry \HKEY_LOCAL_MACHINE\SOFTWARE\WOW6432Node\Pegasus

2. We then using the Microsoft Office Removal Tool (NOT built in program removal) removed the Office product and reinstalled the Office ensuring
    It was 32-Bit
    Version was 1808
    Set to Semi-annual channel
    Desktop Outlook set as default

3. I then reinstalled the Opera3 client and set FULL READ/WRITE permissions on all of the folders 
    c:\programfiles x86\pegasus
    c:\programdata\pegasus
    c:\programfiles x86\common files\pegasus