If you ever delete a user then that account can never be used again or added back. Rather than provide a warning with text that not everyone is going to read it would make more sense to not even have the delete option in the first place or do a soft delete where it can be reactivated. We have employees that have left the company and returned so its not an uncommon scenario. If someone leaves or an account gets deletes accidentally it would be nice to have some sort of recovery option then to not allow that email address to ever be used again if it gets deleted.