When I put in my Cablevision/Optimum email account, I got a message saying "We couldn't find info for that account...." My account ends with @optonline.net.  Cablevision appears to have two servers.  One is the server I use and another ends with @optimum.net.  When I input a my name using @optimum.net, the Windows mail system accepts it though, of course, the Cablevision system does not accept this incorrect email address.  Does Microsoft have to add mail.optonline.net to their database or is there a way to manually set my mail server addresses and ports in Windows 10 mail?