MAPI over HTTP provides the ability for Messaging API (MAPI) clients and servers to communicate across a HTTP connection without using remote procedure calls (RPCs)

The Problems with RPC

Although RPC delivers significant advantages to application developers, it’s an old mechanism that was originally designed to work across LANs rather than across the Internet. 

This is our case

Outlook 2013 or outlook 2010

Outlook Anywhere: users connect to Microsoft Outlook with Outlook Anywhere. Formerly known as RPC over HTTP

New transition is: MAPI over HTTP

 

Two modes of communication that Outlook 2013 SP1 can use with Exchange 2013 SP1

On the left, you have the older RPC-style connections 

On the right, you can see how MAPI over HTTP replaces the dependency on RPC by directing the remote operations executed by Outlook across HTTP connections. This is different than RPC over HTTP because the RPCs used to carry MAPI instructions (wrapped in TCP/IP or HTTP packets) are no longer used. Instead, the MAPI instructions are sent directly over an HTTP link, which is what most Internet applications use to convey their data.

Note: MAPI over HTTP isn’t enabled by default. You have to enable it by updating the organization configuration to let mailbox servers know that it’s OK to tell Outlook clients that MAPI over HTTP is available.

We are using RPC over Http

To enable mapi

http://technet.microsoft.com/en-us/library/dn635177(v=exchg.150).aspx

This is using RPC

http://windowsitpro.com/blog/exchange-2013-mapi-over-http

This is using Mapi

Note MAPI/HTTP connections are designated in the Protocol column using the string HTTP.

 

How does MAPI/HTTP work?

Let’s walk through the scenario of an Outlook 2013 SP1 client connecting to Exchange Server 2013 SP1 after MAPI/HTTP has been enabled.

1. The Outlook client begins with an Autodiscover POST request. In this request Outlook includes a new attribute that advertises the client is MAPI/HTTP capable with the attribute X-MapiHTTPCapability = 1.
   
2. The Exchange server sees the request is coming from a MAPI/HTTP capable client and responds with the MAPI/HTTP information including the settings on how to connect to the mailbox using MAPI/HTTP. This assumes the MAPI/HTTP has been configured and enabled on the server.
   
3. The Outlook client detects the new connection path and prompts the user to restart Outlook to switch to use the new connection. While the restart is pending Outlook will continue using Outlook Anywhere. We recommend you deploy the latest Office client updates to provide the best user experience. The updates remove the prompt and clients are allowed to make the transition at the next unprompted restart of Outlook.
   
4. After the restart, Outlook now uses MAPI/HTTP to communicate with Exchange.
   

   Ref:
   

   http://technet.microsoft.com/en-us/library/dn635177(v=exchg.150).aspx
   

   http://support.microsoft.com/kb/2937684
   

   http://blogs.technet.com/b/exchange/archive/2014/05/09/outlook-connectivity-with-mapi-over-http.aspx
   

   http://windowsitpro.com/blog/exchange-2013-mapi-over-http