I had been using Windows Mail 2012 after upgrading to Windows 10.  Last weeks email telling me that Windows Mail 2012 will not be supported after 6/2016 motivated me to try Windows 10 Mail again.  I have two email account, a live account and a Road Runner email account.  Windows 10 will not add my Road Runner account.  I enter different or POP email to begin to set it up. Enter my email address and password for that account and always get a reply " unable to add this address" with Cancel or Try Again. 

I had my Road Runner email synching with Windows Mail 2012.  Frustrated that Windows Mail 10 won't synch.  I can synch my email addresses on my iPhone but not on Windows Mail 10.......