“Unknown email address. Check again or try your username.”

I understand a user can log in with either username or email. But when username is an email (and particularly different to the actual email) I think it freaks out with above message. I can imagine if it finds an @ symbol it presumes its an email and only looks up email and not username. In my testing, every user that has a different username email to primary email this is a problem. Is there a snippet I can insert to capture this?