Blacklist Check

// What is tested

• One IPv4 address against eight independent DNS blocklists, queried concurrently: Spamhaus ZEN, SpamCop, Barracuda Reputation Block List, SORBS, CBL/abuseat, PSBL, Mailspike and UCEPROTECT level 1.
• Each query reverses the address octets, appends the list's zone and requests an A record. That is the standard DNSBL mechanism.
• Any answer inside 127.0.0.0/8 is treated as a listing, and the returned address is shown so the list's own response code can be interpreted.
• Answers inside 127.255.255.0/24 are treated as error codes, not listings. Those are returned by lists that refuse queries from public resolvers, and the zone is reported as Skipped.

// Data sources

The eight DNSBL zones themselves

zen.spamhaus.org, bl.spamcop.net, b.barracudacentral.org, dnsbl.sorbs.net, cbl.abuseat.org, psbl.surriel.com, bl.mailspike.net and dnsbl-1.uceprotect.net. Each is queried directly; no aggregator sits in between.

DNS over HTTPS

Cloudflare with Google Public DNS as fallback, carrying every zone query.

// Limits

• IPv4 only. DNSBL coverage of IPv6 is inconsistent across providers and is not attempted here.
• Queries reach the lists through a public resolver, and several of them decline those, Spamhaus and CBL in particular. Where that happens the zone is marked Skipped rather than guessed at. Check those lists directly on their own sites.
• Eight lists is not all lists. A receiving mail server may consult others, or maintain its own private reputation data that no public check can see.
• This covers IP-based blocklists only. Domain-based lists (URIBLs), which operate on domains found in message content, are a separate system and are not checked.
• A listing is a snapshot. Lists add and remove entries continuously.