How to configure ISA Server 2004 to allow for RPC over HTTP client connections from Microsoft Office Outlook 2003 to Microsoft Exchange Server 2003: http://support.microsoft.com/?scid=kb;en-us;884506&spid=2108&sid=global
How to publish a Microsoft Exchange server for Outlook Web Access in ISA Server 2004: http://support.microsoft.com/default.aspx?scid=kb;en-us;837354