Test done on new fresh install of DNN220.127.116.116-777
1 - Set "Use Email Address as Username" in Security > Member Accounts > Registration Settings.
2 - Creating a new user using an email address for the username field & Email address gives the following error.
"The username specified is invalid. Please specify a valid username."
3 - Unable to log back into site with either Super User username or Super User email address.