Hello,

If you tried Office 365, you know that you need to login using your UPN, so, if you’re using a non-routable UPN suffix in your domain, you need to add one, an modify users to use this one.Event with that, you may have differences between UPN and email addresses, and causing some confusing to your users. That’s why Microsoft provide us a way to use email addresses as UPN in Office 365 :

http://technet.microsoft.com/en-us/library/dn659436.aspx

This can be useful, and allow us to move to Office 365 transparently for our users.