On the ISA Server 2004, set up an RPC firewall policy to publish the \rpc virtual directory on the RPC proxy server.
See the Microsoft article, Using ISA Server 2004 with Exchange Server 2003 (http://www.microsoft.com/technet/prodtechnol/isa/2004/plan/exchage2003.mspx).
In the walk-through procedures described in the article, follow the instructions in Procedure 8, Configure RPC over HTTP for Outlook 2003.
When you have completed the instructions given in the article, do the following to check that the correct port is specified in the RPC firewall policy for redirected requests.
Check that the correct port is specified in the RPC firewall policy