After installing the update "Update for Publishing Microsoft Exchange Server 2007 for ISA Server 2006" ( http://support.microsoft.com/kb/925403/en-us ), publishing Exchange 2007 using the Exchange RPC Server protocol may fail. Clients may be able to connect
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Read the complete post at http://blogs.technet.com/isablog/archive/2007/12/12/publishing-exchange-2007-may-fail-after-installing-the-update-update-for-publishing-microsoft-exchange-server-2007-for-isa-server-2006.aspx