Starting June 30, 2014, Internet Explorer 8 will support only the light version of Outlook Web App. Changes will take effect during the weeks following June 30th. 

If your organization uses Internet Explorer 8 to access older web apps and services, you might want to upgrade to Internet Explorer 11 and try Enterprise Mode for Internet Explorer 11. Enterprise Mode helps provide better backward compatibility for older web apps. 

For more information we encourage you to visit our web browser guidance at the “Office 365 System Requirements” website: http://technet.microsoft.com/en-us/library/office-365-system-requirements.aspx#BKMK_Web_Browsers