In some situations Outlook 2003 users connecting to an Exchange mailbox using RPC over HTTP receive the following error message:
“Server Unavailable”
Although this is an Outlook 2003 specific client issue, the issue is not specific for Exchange 2010 organizations. It could also appear in organizations running Exchange 2003 or 2007.
The problem occurs if the RPC proxy server extensions do no load correctly. You can find more details and a description of how the issue can be remediated in the following KB article:
Special Thanks to Henrik Walther