[MlMt] Changing default app to open mail messages

[Dave C]

I want to change the email app that opens messages when I click on them 
(as when Spotlight gives one in the results of a search) so MailMate 
opens. Currently Apple Mail opens when I double-click on a message.

I searched the net and see to open Apple Mail (even though that’s not 
the app
I want as a default), open preferences, and change the default in there.

Problem is that the default in the field in Mail preferences says 
“MailMate”. I tried changing it to Mail and back to MM but no joy.

Has anyone had to deal with this before?

Thanks
Dave

MailMate 1.9.6
Mail 10.3
MacOS 10.12.4