The recommended course of action is to store the email in the user.data.email field, which is not required to be unique but is still used for activities like 'forgot password' emails. Leave the user.email field blank and use user.username.
That should work. Support for user.data.email was removed in 1.26 and added back in 1.27.2, so avoid version 1.26.* and 1.27.0 and 1.27.1.