OK, so I've just found one more thing to try, at least it seem to have fixed
things for me. First, make sure you are using Norton's firewall. Then ...
In MS Outlook, go to Tools/Email Accounts, and select Change for the email
account you are having trouble with.
Click the "More Settings ..." button and then the Advanced tab. Click "Use
defaults" then OK.
Not sure why that fixed things since none of the visible settings changed.
But when I tried the "Test Account Settings", it worked!
Hope this works for you.