This has popped up before and must be one of the most common, yet hard to solve problems there is. There are some fixes and downloads at Microsoft but they *mainly don't work*.
Not tech but the easiest way and one which *always works* is....
From the Tools menu, choose Accounts . The Internet Accounts window will appear.
Select the Mail tab.
Select your mail account from the list and choose the Remove button.
Recreate the mail account.
Sometimes you don't need the bells and whistles and fixes and tweaks and hacks....