Windows 8.1 and Windows RT include a built-in email app named Windows Mail. Mail includes support for IMAP and Exchange ActiveSync (EAS) accounts.
This article includes some key technical details of Windows Mail in Windows 8.1. (See Supporting Windows 8 Mail in your organization for Windows 8.0.) Use the information to help you support the use of Mail in your organization. Read this article start to finish, or jump to the topic that interests you. Use the reference links throughout the article for more information
Read more at source: http://blogs.technet.com/b/exchange/archive/2013/10/18/supporting-windows-mail-8-1-in-your-organization.aspx