Mac mail unable to verify Exchange account name or password

I have used Mac mail with my exchange account for several years but it suddely failed to verify the credentials on the exchange server about a week ago. No changes have been made and the same machine is able to log into exchange both through webmail and using the Outlook app.

I've tried removing and re-entering the account both in the mail app and in system preferences - multiple times - without success. Also same process and re-starting laptop.

I've looked at keychain and there are a two lines showing Microsoft Credentials dated today, then two more from 4 days ago. I tried deleting the old ones but it returns an error - invalid record encountered.

Any throughts or recommendations please?