Fix for OWA always uses Light Mode for some users
This article explains the difference between OWA Light Mode and Premium Mode and why some users may only see the Light Mode client, even though they haven't selected it at logon. Exchange 2007 Outlook Web Access and Exchange 2010 Outlook Web App offer
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Read the complete post at http://www.expta.com/2010/08/fix-for-owa-always-using-light-mode-for.html