Windows 10, Office 365(Outlook 2016) and Default mail app

I have just set up a new Windows 10 Pro workstation.  I have installed Office 365 (all apps), Sage 50 (our accounting software) and various other apps.  
Everything seemed to be working well until the user attempted to email an invoice to a customer using Sage.  We got an error message stating that there was no default email app installed.
I thought this was strange since the email had been working fine.  However when I looked at the "Default" apps in the settings, I saw that the default mail app was set to "Mail".   "Outlook 2016" was in the drop down list but when I try to change the default it won't change from "Mail".  
I have changed the user from "user" to "local administrator" with no change.  I have deleted the account and rebuilt with a new profile with no luck.

I'm really kind of stuck as this is a strange problem and other then deleting Office 365 and reinstalling I'm at a dead end.
Any ideas would be appreciated.

Ken