Office 365 will no longer support Internet Explorer 8 after 8 April 2014.
For the best, richest experience with Office 365, Microsoft recommends that you upgrade to the latest version of Internet Explorer. Internet Explorer 10 is available now, and Internet Explorer 11 is coming soon. Modern versions of Internet Explorer offer faster web browsing, integrated spell-checking, improved security, and more.
Internet Explorer 8 users will not be blocked from connecting to Office 365 after 8 April 2014; however, Internet Explorer 8 users should expect slower performance with Office 365’s Outlook Web App and other Office Web Apps.