Messaging Application Programming Interface (MAPI) over HTTP is a new transport protocol implemented in Microsoft Exchange Server 2013 SP1 and Microsoft Outlook 2013 SP1. MAPI over HTTP improves the reliability and stability of the Outlook and Exchange connections by moving the transport layer to the industry-standard HTTP model. This new connection method has been gradually rolling out in Office 365 since May of 2014/ The following blog post provides great coverage of MAPI over HTTP: http://blogs.technet.com/b/exchange/archive/2014/05/09/outlook-connectivity-with-mapi-over-http.aspx