Validates a "new" email address by checking if it is already used by other users.

def validate_new_email(email):
    """Validates a "new" email address by checking if it is already used by other users."""
    email = normalize_email(email)
    User = get_user_model()  # noqa
    if User.objects.filter(email=email).exists():
        raise ValidationError('The given email address is already registered.')