@classmethod
def normalize_email(cls, email):
    """
        Normalize the email address by lowercasing the domain part of it.
        """
    email = email or ''
    try:
        email_name, domain_part = email.strip().rsplit('@', 1)
    except ValueError:
        pass
    else:
        email = email_name + '@' + domain_part.lower()
    return email
