unable to verify account name or password

Have you installed and run any "cleaning", "optimizing", "speed-up", anti-virus or VPN apps on your Mac?

Give this a try: boot into Safe Mode according to How to use safe mode on your Mac and test to see if the problem persists. Reboot normally and test again.

NOTE: Safe Mode boot can take up to 3 - 5 minutes as it's doing the following; 

• Verifies your startup disk and attempts to repair directory issues, if needed

• Loads only required kernel extensions (prevents 3rd party kernel/extensions from loading)

• Prevents Startup Items and Login Items from opening automatically

• Disables user-installed fonts 

• Deletes font caches, kernel cache, and other system cache files

Try using an app specific password. See the earlier discussion on the subject.

App specific password solves MS Outlook m… - Apple Community

jaydog_1973 wrote:

Could it be a Microsoft issue?

Isn't everything an MS issue? 😉 That's an MS Outlook account, right? There has been some other reports of problems with Outlook. Do a search in these forums for "Outlook" and see what you find. I didn't pay any attention to what was said.

Try removing and recreating the mail account from scratch. Apparently from the postings here, it's not that unusual for this to happen with a change in OS.