RPC over HTTP (Remote Procedure Call over HTTP) is a protocol that
allows a client on the Internet to connect securely to a Microsoft
Exchange Server without having to log into a virtual private network (VPN) first.
RPC over HTTP uses Secure Socket Layer (SSL) as a transport protocol and mandates that the server authenticates itself to the client using a digital certificate and associated private key. The protocol takes advantage of the Hypertext Transfer Protocol (HTTP) connectivity built into the Outlook Web Access (OWA), foregoing requirements for an administrator to open additional ports on the corporate firewall.
http://msdn.microsoft.com/en-us/library/windows/desktop/aa375384%28v=vs.85%29.aspx
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