Default mail client - Reader 19.021.20058

I have a unit that was recently upgraded in place from Win7 to Win10.  User is complaining that his default mail client is now set to Outllook (which he does not use).  Going into Prefs > EmailAccts

shows :

Account

    Default email application (Microsoft Outlook)

Default mail application on this PC is Thunderbird.

My personal PC (and others here) which were upgraded at the same time show:

Account

    Default mail application

Approx. 40% of the other PC's that have been upgraded here aslo suffer from this ailment.

All of these PC's are set to Thunderbird as default mail app.

Is there anyway to get Acrobat to "let go" of it's attachment to Outlook?

Tks