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