MAPI over HTTP (Messaging Application Programming Interface over HTTP)
MAPI over HTTP is the default transport protocol to connect clients to Microsoft Exchange and Exchange Online.
The Messaging Application Programming Interface (MAPI) over Hypertext Transfer Protocol (HTTP) supersedes the Remote Procedure Call (RPC) over HTTP protocol, which is also known as Outlook Anywhere. Microsoft introduced MAPI over HTTP in Exchange 2013 SP1 and Outlook 2013 SP1 in May 2014, then released it for use with Microsoft's hosted messaging platform Exchange Online -- available as a standalone product or with an Office 365 plan.
The advantage of MAPI over HTTP is its relative simplicity. RPC over HTTP wrapped its MAPI payload within both RPC and HTTP for transport over the network. MAPI over HTTP just requires an HTTP wrapper. This reduction eliminates the need for two long-term TCP connections for each Outlook or Exchange session and connections for RPC data. MAPI over HTTP uses two TCP connections -- one long-term connection and one short-term connection -- for more efficient network use and improved email performance. A lost, stopped or changed network connection under RPC over HTTP requires the creation of a new RPC connection; MAPI over HTTP reconnects more quickly with just a TCP connection and can resume where it stopped. MAPI over HTTP can also reduce network issues when a server-side error disconnects multiple users who try to reconnect simultaneously.
There are several prerequisites needed to support MAPI over HTTP with Exchange Server 2016. For email clients, organizations can use Outlook 2016, Outlook 2013 SP1 (or later), or Outlook 2010 SP2 (with KB2956191 and KB2965295 updates). Organizations also need to run Exchange 2016 on Windows Server 2012 R2 or later. Administrators will need to upgrade older Windows Server versions to Microsoft .NET Framework to 4.5.1 (or later) along with a hotfix rollup for .NET Framework 4.5.1 on Windows Server 2008 R2 SP1 (KB2908383), Windows Server 2012 (KB2908385) or Windows Server 2012 R2 (KB2908387).
Microsoft enables MAPI over HTTP by default in Exchange Server 2016, but administrators must also configure the platform's virtual directories. Organizations that upgrade to Exchange Server 2016 or run a mix of Exchange 2010 and Exchange 2016 servers will also need to ensure a proper configuration before using MAPI over HTTP. Exchange Server 2013 supports MAPI over HTTP, but administrators must enable and configure the server to use the protocol.
The use of MAPI over HTTP does not prohibit Outlook clients from using other protocols. For example, older Outlook clients can use RPC over HTTP to communicate with an Exchange Server configured for MAPI over HTTP.
In 2016, Microsoft announced it will end support for RPC over HTTP in Outlook to access Exchange Online mailboxes in favor of MAPI over HTTP after October 31, 2017. The company said older versions of Outlook, such as Outlook 2007, that use RPC over HTTP will not be able to connect to Exchange Online after that date.