Microsoft Knowledge Base Article 884506 contains the following INTRODUCTION:

This step-by-step article describes how to configure Microsoft Internet Security and Acceleration (ISA) Server 2004 so that Remote Procedure Call (RPC) over HTTP client connections can pass from Microsoft Office Outlook 2003 to Microsoft Exchange Server 2003. The RPC over HTTP feature makes it possible for remote client computers that are running Microsoft Windows Server 2003, Microsoft Windows XP Service Pack 1, or later versions of Microsoft Windows, to connect to an Exchange server without the need for a virtual private network (VPN) connection. You can configure the RPC over HTTP traffic to be sent through an ISA Server 2004-based computer on your network by creating a Secure Sockets Layer (SSL) Web Publishing rule on the ISA Server 2004-based computer.

Note You must confirm that RPC over HTTP traffic is functioning correctly on the Exchange server before you configure RPC over HTTP traffic to pass through the ISA Server 2004-based computer.

For more information about how to deploy Exchange Server 2003 RPC over HTTP, click the following article numbers to view the articles in the Microsoft Knowledge Base:

840255 The "Exchange Server 2003 RPC over HTTP deployment scenarios" guide is available

833401 How to configure RPC over HTTP in Exchange Server 2003

827330 How to troubleshoot client RPC over HTTP connection issues in Office Outlook 2003

For more information about Exchange Server 2003 RPC over HTTP, visit the following Microsoft Web sites:

Exchange Server 2003 RPC over HTTP Deployment Scenarios
   http://www.microsoft.com/downloads/details.aspx?FamilyID=f7d2d6e5-579f-4779-a6b8-7ef931ec02a5&DisplayLang=en

Remote Procedure Calls Using RPC over HTTP
   http://msdn.microsoft.com/library/default.asp?url=/library/en-us/rpc/rpc/remote_procedure_calls_using_rpc_over_http.asp

Configuring Outlook 2003 for RPC Over HTTP
   http://www.microsoft.com/office/ork/2003/three/ch8/OutC07.htm