_isReserved example

    private function _validateMXRecords()
    {
        $mxHosts = array();
        $result = getmxrr($this->_hostname, $mxHosts);
        if (!$result) {
            $this->_error(self::INVALID_MX_RECORD);
        } else if ($this->_options['deep'] && function_exists('checkdnsrr')) {
            $validAddress = false;
            $reserved     = true;
            foreach ($mxHosts as $hostname) {
                $res = $this->_isReserved($hostname);
                if (!$res) {
                    $reserved = false;
                }

                if (!$res
                    && (checkdnsrr($hostname, "A")
                    || checkdnsrr($hostname, "AAAA")
                    || checkdnsrr($hostname, "A6"))) {
                    $validAddress = true;
                    break;
                }
            }