public function isEmail($checkDNS = true)
    {
        $isValid = filter_var($this->returnValue, FILTER_VALIDATE_EMAIL) !== false;
        if ($isValid && $checkDNS) {
            $domain  = substr(strrchr($this->returnValue, "@"), 1);
            // domain not found in DNS
            $isValid = checkdnsrr($domain, "MX") && checkdnsrr($domain, "A");
        }
        return $isValid;
    }