Technical Details of Using RPC over HTTP to Access Exchange from an Outlook Client
RPC over HTTP allows Microsoft® Office Outlook® 2003 clients to access Microsoft Exchange servers by using the MAPI protocol to tunnel Outlook RPC requests inside an HTTP session, or tunnel. Typically, an Outlook 2003 client contacts an Exchange server over a TCP session.
If you use RPC over HTTP, you can use Outlook 2003 to access Exchange Server over the Internet, because HTTP is a common Internet protocol. The HTTP session terminates at a server running Internet Information Services (IIS) that has the Microsoft Windows Server™ 2003 RPC over HTTP Proxy networking component installed. This server is called an RPC proxy server.
It is recommended that your RPC proxy server is an Exchange front-end server. The RPC over HTTP Proxy networking component extracts the RPC requests from the HTTP request and forwards the RPC requests to the appropriate server. The advantage of this approach is that only the RPC proxy server has to allow access from the Internet. Back-end Exchange servers do not have to allow access from the Internet. You should use the Secure Sockets Layer (SSL) to establish the HTTP session that you use to access Exchange Server over the Internet from an Outlook 2003 client.
A common misconception is that the use of RPC over HTTP turns a Web request into an RPC request. Outlook 2003 sends the RPC request inside an HTTP tunnel. The actual RPC request does not change between the Outlook client and the Exchange server.
The following figure shows an RPC request inside an HTTP tunnel.
RPC request inside HTTP tunnel