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Microsoft Office Outlook 2007 and Microsoft Office Outlook 2003 support only the following two remote procedure call (RPC) bindings when Outlook connects to a server that is running Microsoft Exchange Server:
RPC protocol string
Outlook 2007 and Outlook 2003 can connect to a Microsoft Exchange server only by using either RPC over TCP/IP or RPC over HTTP. Outlook 2007 and Outlook 2003 do not try to use named pipes or any other RPC binding method to establish a connection to an Exchange server.