Do you have a subscription and a MS account? There should be a cloud backup.

While I am not expert at these things, I am thinking you might still have the old account data on the computer. If that is so, then going into the outlook settings where email accounts are configured would allow changing it back.

I use M365 at work and a little at home. It is well behaved in a corporate environment. Don't use outlook at home so I can't help too much there.

Office apps are ‘different’ but I’d be surprised if an update managed to delete your outlook data.

I know it's some setting issue, but I deliberately put in typos, and it didn't catch one. I have told my students not to use it because it will give them false confidence.

I still have a Word 2003 version that has a better editor!

Outlook is run by Microsoft, and is obviously suffering from whatever the "update" did to authentication.

Perhaps that wouldn't have happened to Thunderbird. MS supports IMAP, IIRC.

My brother had to reinstall peripheral drivers every month because patches killed them like clockwork. He threw in the towel and ended up with an Ubuntu Linux system sold and supported by Dell, and hasn't called me for help in years.

He uses outlook.com (MS) as a mail server.

it is complete crap. Yet every MSCE (Microsoft Click Engineer) pushes their client or customer to use it.
Microsoft loves it. Now not only do you not (or ever) get a set of discs with your so called copy of Office, Microsoft completely controls your data, shoves ads down your throat, can change the file format when ever they want and yes......even gets to charge you forever for the privilege of using this garbage. Add in the regular security issues of using a online hosted solution like Office 365 and you have garbage on top of garbage.
As others have mentioned there are numerous alternatives out there. LibreOffice is a great office solution and Thunderbird works nicely as an email client. Both are cross platform (run in Linux and Windows) leaving the door open for an escape from the Microsoft garbage pit.