Consider the following scenario:
You are using Exchange Online/Office 365. When opening Outlook Web App (OWA), Internet Explorer only loads the Light version of OWA (OWA Light). The same user on the same client could earlier load the rich OWA experience even on Internet Explorer 8.
If the same user logs on to Internet Explorer 9 or later, or the latest version of Firefox, Chrome or Safari, rich OWA is loaded instead. OWA Light is only enforced on Internet Explorer 8.
This behavior is by design. Rich OWA have earlier been allowed for Internet Explorer 8 users, but with limited performance (see this article). Starting May 2014, OWA Light is being enforced for Internet Explorer 8 users. This is stated in the Office 365 Service Description, see this article.
Quote from the Service Description:
“The user experience sending and receiving email with Outlook Web App and Internet Explorer 8 might be substantially diminished, especially when used on Windows XP or with low memory devices. Office 365 does not offer code fixes to resolve problems you encounter when using the service with Internet Explorer 8, and new Office 365 experiences might not work at all. You should also expect the quality of the user experience with Internet Explorer 8 to diminish further in the near future. Beginning in May 2014, Internet Explorer 8 will only display Outlook Web App Light.”
Upgrade Internet Explorer to the latest version, or use the latest version of an alternative browser. If you have legacy application dependencies to Internet Explorer 8, consider upgrading to Internet Explorer 11 and use IE Enterprise Mode to emulate IE8 rendering for those legacy web sites/applications.
And remember – Office 365 is an “evergreen” service, meaning that back-end updates will be released constantly. As a customer, you should keep your client applications on the latest version/build to stay fully compatible and supported. This includes Windows, Office applications and Internet Explorer.