Using Classic Outlook on Windows for personal email, calendar, and contact management
I figured it out.
In Apple Mail I mistakenly used the 'Other Mail Account...' option when I installed the email there a few years back.
It started having troubles in the last couple of months and now won't work since MS shut down using the app passwords for it yesterday without a note at that page about it.
I SHOULD HAVE installed the email as a 'Microsoft Exchange' email account. Which I just did. It works great.