RPC data may be blocked, and Outlook may not start in Windows Server 2003 with SP1
http://support.microsoft.com/?id=897716

SYMPTOMS
  On a computer that is running Microsoft Windows Server 2003 with Service Pack (SP1), remote procedure call (RPC) data may be blocked by a Microsoft Internet and Acceleration (ISA) Server 2004 server or an ISA 2000 server that resides between the local computer and an Exchange server. When this problem occurs, Outlook may not start.

CAUSE
  This problem occurs because the RPC filter in ISA Server is not compatible with the changes to the RPC protocol in Windows Server 2003 SP1.   When you enable the RPC filter in ISA Server, the RPC filter enforces strict compliance with a subset of the RPC protocol that is used by Microsoft Exchange Server MAPI communications.

Download für den SBS 2003:
http://www.microsoft.com/downloads/details.aspx?displaylang=de&FamilyID=0b3b8b50-9a7a-43ca-8e30-ccfcd8ad5fa3

More: http://msmvps.com/bradley/archive/2005/05/26/49121.aspx

Tobias Redelberger

StarNET Services (HomeOffice)
Schoenbornstr. 57
D-97440 Werneck

   Tel:   +49-(0)9722-4835
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Email:   T.Redelberger@starnet-services.net