function _getMx($host)
    {
        $mx = array();

        if ($this->withNetDns) {
            $res = $this->_loadNetDns();
            if (is_a($res, 'PEAR_Error')) {
                return $res;
            }

            $response = $this->resolver->query($host, 'MX');
            if (!$response) {
                return false;
            }

            foreach ($response->answer as $rr) {
                if ($rr->type == 'MX') {
                    $mx[$rr->exchange] = $rr->preference;
                }
            }
        } else {
            $mxHost = array();
            $mxWeight = array();

            if (!getmxrr($host, $mxHost, $mxWeight)) {
                return false;
            }
            for ($i = 0; $i < count($mxHost); ++$i) {
                $mx[$mxHost[$i]] = $mxWeight[$i];
            }
        }

        asort($mx);
        return $mx;
    }