Mail on my Pro suddenly says incorrect username and/or password. Resubmitting password does not help. Mail is fine on my iPad and Phone. I can also log into the mail's website with no issue. Until now mail has never been an issue on my Mac.
I tried deleting and re-adding the account. Get the same message when it tries to verify the login during setup.
I hope you solved your problem. I now have the same thing happening on my ipad. Used to get and send mail, all of a sudden I can only receive mail, when I try to send it says my user name or password are incorrect.