When you try to access Outlook Web App (OWA) Premium by using Internet Explorer 11 in a Microsoft Exchange Server 2013, Microsoft Exchange Server 2010, Microsoft Exchange Server 2007, or Microsoft Exchange Server 2003 environment, OWA Light is loaded instead.
To work around this issue in Exchange Server 2013, Exchange Server 2010, Exchange Server 2007, or Exchange Server 2003, use Internet Explorer 11 in compatibility mode to access OWA. To do this, press F10 to display the menu bar, go to the Tools menu in Internet Explorer 11, and then click Compatibility View settings. Then, add the OWA site to the list of sites to be viewed in compatibility view.
See article: http://support.microsoft.com/kb/2866064/en-us