Same here...as I can't find a way to rename these horrible email addresses used as account names, I'll be using Thunderbird too.
Microsoft have tried to shove "New Outlook" down to our throat since each time I've been starting "Mail for Windows" (which DOES work far better, and you can actually change the displayed account name) Mail App is now shutting down and instead the Mail app starts in "New Outlook" mode -of course without asking.
While the "new outlook" toggle was still appearing, I gave feedback to Microsoft dozens of times reporting the loss of functionality. Did they bother to fix it? Of course not! So Thunderbird it is for me till they start respecting their customers.