Decided to do a quick test before going to bed. Created a new Outlook email account using my business name. Then I set that new account Data File in Outlook to be the default, leaving my business Gmail account to be the default email address. Closed Outlook, then re-opened it to activate the change. Went back to TaxCycle and printed a client E-file Receipt, and voila, it opened Outlook using the new Business Outlook address.
So problem solved. It is the Outlook Data File that controls which email address is selected in the outgoing email message sent by TaxCycle. It seems that only Outlook data files can be accepted as defaults. Gmail email addresses aren’t compatible.