def validate_domain(self, part):
        part, err = self._apply_common_rules(part, maxlength=255)
        
        if err:
            return part, 'Invalid domain: %s' % err
        
        if not self.domain_regex.search(part):
            return part, 'Invalid domain.'
        
        if self._lookup_dns and not self.lookup_domain(part):
            return part, 'Domain does not seem to exist.'
        
        return part.lower(), ''
dataset/ETHPy150Open marrow/mailer/marrow/mailer/validator.py/DomainValidator.validate_domain/original