protected function validate_valid_email($field, $input, $param = null)
    {
        if (!isset($input[$field]) || empty($input[$field])) {
            return;
        }

        if (!filter_var($input[$field], FILTER_VALIDATE_EMAIL)) {
            return array(
                'field' => $field,
                'value' => $input[$field],
                'rule' => __FUNCTION__,
                'param' => $param,
            );
        }
    }