DNSBL Check

What a DNSBL Is

A DNS Blacklist (DNSBL or RBL) is a list of IP addresses distributed via DNS. Mail servers and spam filters query these lists in real time when deciding whether to accept mail from a given sending IP. The query is just a regular DNS A-record lookup: the bytes of the IP are reversed and appended to the list's domain. If the list returns an A record (usually a 127.x.x.x code), the IP is listed; if it returns NXDOMAIN, it's clean. DNSBLs are one of the cheapest and most effective spam defenses still in use.

Which Lists This Tool Queries

The tool checks 12 publicly-accessible lists operated by SpamCop, Barracuda, PSBL, UCEPROTECT, Mailspike, DroneBL, SpamRats, s5h.net, Blocklist.de, InterServer, Manitu (NiX Spam), and GBUdb. These are all lists that respond to queries from public DNS resolvers with real answers.

Why Spamhaus, CBL, and SORBS Are Not Included

Spamhaus (ZEN, SBL, XBL, DBL) explicitly blocks queries that come from public DNS resolvers like Cloudflare 1.1.1.1 and Google 8.8.8.8. When queried through those resolvers they return a sentinel code (127.255.255.254) meaning "not answered because the query came from a public resolver." If this tool included Spamhaus, every query would incorrectly appear listed. Because this tool runs client-side through Cloudflare DoH, there is no way to include Spamhaus reliably — use their own lookup at spamhaus.org/check for authoritative results. CBL was merged into Spamhaus XBL and has the same limitation. SORBS was excluded because of reliability problems since its 2023–2024 ownership changes.

What to Do If You’re Listed

First, figure out why before requesting delisting. Common reasons are: a compromised server sending spam, a misconfigured mail form or contact page being abused as a relay, an application sending high-volume unsolicited email, or a machine infected with malware. Delisting before fixing the root cause usually just gets you relisted within hours. Click a list name in the results to open the operator's own delisting portal. If you're on a shared hosting IP or a cloud provider's mail IP, you may need the provider to request delisting on your behalf.

Need Help?

I handle mail deliverability investigations, root-cause analysis, and post-incident cleanup for clients running their own mail infrastructure. If you’re stuck in a cycle of getting listed, delisted, and relisted, there’s almost always an upstream cause that needs fixing. Get in touch. See also the Email Auth Checker and Reverse DNS + FCrDNS Checker.