The User().Email function is actually returning the UPN and not the user's email address. Most people don't notice this issue because their UPN and Email Address are the same. In our tenant our UPN and Email Address are very different. Since the User().Email returns user's UPN not their email address we are unable to use this function in PowerApps. It would be nice to get this issue fixed. The work around is to use the Office365Users connector to get the current users profile and get the email address from there.