Windows 8 Mail requires 3rd party SSL certificate for Exchange
If you have a Windows 8 Mail client that won't connect to an Exchange account, check to see if you have a self-assigned SSL certificate installed on your Exchange Server. A "3rd party" certificate (GeoTrust, Thawte, VeriSign, Go Daddy for example) will take care of your problem. You can confirm if this is in fact the case by running the Microsoft Remote Connectivity Analyzer, testing Exchange ActiveSync (https://www.testexchangeconnectivity.com/).