// Quote: SMF; [SiNaN]
function domain_exists($email, $record = 'MX')
{
    list($user, $domain) = split('@', $email);

    $function = (function_exists('checkdnsrr') ? '' : 'smf_') . 'checkdnsrr';

    return $function($domain, $record);
}

function smf_checkdnsrr($host, $type = '')
{
    if (!empty($host))
    {
        if ($type == '')
            $type = "MX";

        exec("nslookup -type=$type $host", $result);
        foreach ($result as $line)
        {
            if (preg_match('~^' . preg_quote($host) . '~i', $line))
                return true;
        }
        return false;
    }
    return false;
}

// Örnek: domain_exists('ali@ata.bak')