Mail is "Unable to verify account name or password."

I've got a Mac Pro (mid 2010) running macOS High Sierra (v10.13.6).

I know. Very old. But, it's what I've got and it runs most everything fine.

So, the other day Mail surprised me by asking for a password on the email account I use most. It's been on this computer for years and years. I'm not aware of having changed anything (though it's set to automatically update).

I use the same account with the same password on two iPads, my iPhone and my 17" MacBook Pro. They're still accessing my email fine. But when I typed in the password I got a text line that says, "Unable to verify account name or password."

I tried going in through "Accounts." Same thing.

As you can see, it's an Exchange account and I verified the password with my email provider.

I've searched all over and can't find anyone with a similar problem. I've tried turning the account off and on. Tried turning off the account, deleting all preferences and restarting the computer.

I thought I'd delete the account and start over, adding it to this computer again but this dialog box scared me off that idea. Like I said, it's working fine on my other devices.

So I just tried turning off the account and adding the Exchange account again. That didn't work.

I've looked for instructions to uninstall and reinstall Mail but most forums say it's not possible/advisable.

Any ideas what to try next?

Thanks in advance for your time.

Processor 2 x 2.66 GHz 6-Core Intel Xeon with 16 GB 1333 MHz DDR3.