data.netsyms.net/endpoints/net.contactspam.php

<?php

// Minimum message length checks
const MSG_MIN_CHARS = 20;
const MSG_MIN_WORDS = 5;
// Banned words check, file should be one match (word or phrase) per line
const BANNED_WORDLIST = __DIR__ . "/../resources/net.contactspam/bannedwords.txt";
// Banned email domain check, one domain per line, useful if you get lots of spam from
// a domain your customers probably won't legitimately use
const BANNED_DOMAINS = __DIR__ . "/../resources/net.contactspam/banneddomains.txt";
const BANNED_IP_LIST = __DIR__ . "/../resources/net.contactspam/bannedips.txt";
const BANNED_IP_CIDR = __DIR__ . "/../resources/net.contactspam/toxic_ip_cidr.txt";

$message = $VARS["message"] ?? "";
$fromemail = $VARS["email"] ?? "";
$clientip = $VARS["ipaddr"] ?? "";
$contactformdomain = trim(strtolower($VARS["domain"] ?? ""));

$msg_lower = trim(strtolower($message));
$email_lower = trim(strtolower($fromemail));
$email_parts = explode("@", $email_lower);
$email_domain = $email_parts[count($email_parts) - 1];

//
// If message too short (chars and/or words)
//
if (isset($VARS["message"])) {
    if (strlen($msg_lower) < MSG_MIN_CHARS) {
        exitWithJson(["status" => "OK", "clean" => false, "filter" => "length_chars", "hit" => "", "message" => "Message too short."]);
    } else if (str_word_count($msg_lower) < MSG_MIN_WORDS) {
        exitWithJson(["status" => "OK", "clean" => false, "filter" => "length_words", "hit" => "", "message" => "Message too short."]);
    }
}


//
// Check email domain
//
$banneddomainlist = file(BANNED_DOMAINS, FILE_IGNORE_NEW_LINES);
foreach ($banneddomainlist as $domain) {
    if ($email_domain == $domain) {
        exitWithJson(["status" => "OK", "clean" => false, "filter" => "domain", "hit" => $domain, "message" => "Emails from \"" . htmlspecialchars($domain) . "\" are not allowed because of spam/abuse." . ($domain == "googlemail.com" ? "  (Hint: use gmail.com instead)" : "")]);
    }
}

//
// Check if email address is sketchy
//
if (!empty($email_parts) && count($email_parts) == 2) {
    if ($email_parts[0] == $email_parts[1]) {
        exitWithJson(["status" => "OK", "clean" => false, "filter" => "email_fake", "hit" => "", "message" => "Unacceptable email address."]);
    }
}
if (!empty($contactformdomain)) {
    if ($contactformdomain == $email_domain && $email_parts[0] != "test") {
        exitWithJson(["status" => "OK", "clean" => false, "filter" => "email_self", "hit" => "", "message" => "Please use your own email address, not ours."]);
    }
}


//
// Check for blocked spammy words
//
$bannedwordlist = file(BANNED_WORDLIST, FILE_IGNORE_NEW_LINES);
foreach ($bannedwordlist as $word) {
    if (strpos($msg_lower, $word) !== false) {
        exitWithJson(["status" => "OK", "clean" => false, "filter" => "word", "hit" => $word, "message" => "\"" . htmlspecialchars($word) . "\" is not allowed because of spam/abuse."]);
    }
}

//
// Lookup reported client IP address against stopforumspam.com CIDR range list
//
/**
 * https://stackoverflow.com/a/594134
 */
function cidr_match($ip, $range) {
    list ($subnet, $bits) = explode('/', $range);
    if ($bits === null) {
        $bits = 32;
    }
    $ip = ip2long($ip);
    $subnet = ip2long($subnet);
    $mask = -1 << (32 - $bits);
    $subnet &= $mask; # nb: in case the supplied subnet wasn't correctly aligned
    return ($ip & $mask) == $subnet;
}

if (filter_var($clientip, FILTER_VALIDATE_IP, [FILTER_FLAG_IPV4])) {
    $bannedipcidrlist = file(BANNED_IP_CIDR, FILE_IGNORE_NEW_LINES);
    foreach ($bannedipcidrlist as $cidr) {
        if (cidr_match($clientip, $cidr)) {
            exitWithJson(["status" => "OK", "clean" => false, "filter" => "toxic_ip_cidr", "hit" => $clientip, "message" => "Your computer's IP address is on a spam blacklist."]);
        }
    }
}

//
// Lookup reported client IP address against stopforumspam.com full IP list
//
if (filter_var($clientip, FILTER_VALIDATE_IP, [FILTER_FLAG_IPV4])) {
    $bannediplist = file(BANNED_IP_LIST, FILE_IGNORE_NEW_LINES);
    foreach ($bannediplist as $ip) {
        if ($clientip == $ip) {
            exitWithJson(["status" => "OK", "clean" => false, "filter" => "banned_ip", "hit" => $clientip, "message" => "Your computer's IP address is blacklisted for sending spam."]);
        }
    }
}


//
// Lookup reported client IP address to see if it's in a botnet or something and sending spam
//
try {
    if (filter_var($clientip, FILTER_VALIDATE_IP, [FILTER_FLAG_IPV4])) {
        $blacklist = "xbl.spamhaus.org";
        $url = implode(".", array_reverse(explode(".", $clientip))) . "." . $blacklist;

        // Cache IPs so we don't do a DNS lookup each time
        $cacheresp = $memcache->get("net.contactspam.$url");
        if ($cacheresp !== false) {
            $dns_result = $cacheresp;
        } else {
            $dns_result = `host -t A $url b.gns.spamhaus.org`;
            $memcache->set("net.contactspam.$url", $dns_result, 60 * 60 * 24);
        }
        if (strpos($dns_result, "NXDOMAIN") === false && strpos($dns_result, "127.0.") !== false) {
            exitWithJson(["status" => "OK", "clean" => false, "filter" => "spamhaus_xbl", "hit" => $clientip, "message" => "Your computer or a device on your network is infected with a virus and is sending spam messages, so our system is blocking your message."]);
        }
    }
} catch (Exception $ex) {

}

//
// Well if we got here then the message tested negative for spam
//
exitWithJson(["status" => "OK", "clean" => true, "filter" => null, "hit" => null]);