'Normalize the email address by lowercasing the domain part of it.'

@classmethod
def normalize_email(cls, email):

    email = (email or '')
    try:
        (email_name, domain_part) = email.strip().rsplit('@', 1)
    except ValueError:
        pass
    else:
        email = ((email_name + '@') + domain_part.lower())
    return email
