Apple Mail - 'Unable to verify account name or password'

I resolved this by instead completely reconnecting the account. On Sonoma:

1. Go to the Mail app (top right)
2. Settings -> Accounts
3. Click the "+" sign in the bottom of the window that pops up
4. Click "Microsoft Exchange"
5. Go through the login steps and connect the account. As a side note you can rename your new account to whatever your old one is by clicking on it, highlighting the "Account Information" tab and editing the "Description" field
6. Remove the old account by clicking on it and then clicking the "-" button on the bottom of the window. Follow prompts to remove that stale account.