$parts = parse_url($url);
$host = $parts['host'];

# Find MX record to use for SURBL
$pieces = explode('.', $host);
$mx_found = 0;

for ($i = 2; $i <= count($pieces); $i++) {
	$possible_domain = implode('.', array_slice($pieces, -$i));
	if (!checkdnsrr($possible_domain, 'MX')) {
		continue;
	} else {
		$mx_found++;
		break;
	}
}

# Reject domains without MX
if (!$mx_found) {
	return "NOMX";
} 

# Check SURBL
if (checkdnsrr("$possible_domain.multi.surbl.org", 'A')) {
	return "SURBL";
}