Windows Security pop-up

Checking email as usual, then a Windows Security window pops up, asking for
my mail server User Name and Password.
So I input (but apparently incorrect).

I tried going to Windows Security, and Windows Mail to try to get rid of it.
I think it must be new since my Windows upgrade last night . ARGH!!!

How do I get rid of this?
(I wish I could put a screen shot to show).