How does a DNSBL lookup actually work?

The IP's octets are reversed (203.0.113.10 becomes 10.113.0.203) and queried as a subdomain of the blacklist's zone, like 10.113.0.203.zen.spamhaus.org. An A record answer, almost always in the 127.0.0.x range, means the IP is listed; no answer means it isn't.

What does 'Query blocked' mean, instead of listed or not listed?

This tool queries through public DNS-over-HTTPS resolvers rather than your own DNS server, and Spamhaus in particular reserves the 127.255.255.0/24 range of return codes to mean 'this query was rejected', most often because it arrived via a well-known public resolver IP rather than a real listing lookup. This tool detects those specific codes and reports them as blocked rather than mislabeling them as an actual listing.

My mail server's IP is listed. What do I do?

Each blacklist operator publishes its own delisting process, usually a web form on their site, once you've fixed the underlying spam or abuse issue. This tool only reports the current listing status; it can't request removal for you.

DNS Blacklist Checker - Check an IP Against DNSBLs

Overview

Introduction

Mail servers commonly reject incoming connections from IPs listed on public DNS-based blacklists (DNSBLs) before even looking at the message content.

This tool runs the same kind of lookup those mail servers do, checking an IPv4 address against several well-known blacklists at once.

What Is DNS Blacklist Checker?

A blacklist lookup tool that checks an IP against Spamhaus ZEN, SpamCop, SORBS, and Barracuda using the standard DNSBL query format.

It's part of this site's DNS & Domain Tools collection; each check is a real, live DNS query.

How DNS Blacklist Checker Works

The IP's four octets are reversed and appended to each blacklist's zone name to form a query, following the DNSBL convention every major blacklist uses.

If that query returns an A record, the IP is listed on that zone; if it returns no answer, it isn't (at least as seen by this lookup path).

When To Use DNS Blacklist Checker

Use it when outbound mail from an IP starts bouncing or landing in spam, to check whether the IP has landed on a well-known blacklist.

It's also useful before provisioning a new server IP, to check whether a previous tenant left it with a bad reputation.

Often used alongside MX Record Lookup and MTA-STS Checker.

Features

Advantages

Checks four widely-used blacklists in one pass instead of looking each one up separately.
Uses the exact same DNS query mechanism real mail servers use to make blocking decisions.

Limitations

Queries go through public DoH resolvers, and some operators (notably Spamhaus) rate-limit or block lookups from well-known public resolver IPs, so results from those zones aren't fully conclusive.
IPv6 addresses aren't supported; DNSBLs overwhelmingly still key off reversed IPv4 octets.

Examples

Checking a clean IP

Input

8.8.8.8

Output

Spamhaus ZEN: Not listed
SpamCop: Not listed
SORBS: Not listed
Barracuda: Not listed

No zone returned an A record for the reversed-IP query, so the address isn't currently listed on any of them.

Best Practices & Notes

Best Practices

Check a new server or SMTP relay IP before putting it into production, especially on cloud providers where IPs get reassigned between customers.
If an IP is listed, fix the root cause (open relay, compromised account, spam campaign) before requesting delisting; most operators re-list quickly if the underlying issue isn't resolved.

Developer Notes

Only IPv4 is supported since virtually every production DNSBL still keys off reversed dotted-quad octets; IPv6 DNSBLs exist but are rare enough that supporting them would mostly add unused UI.

DNS Blacklist Checker Use Cases

Diagnosing mail deliverability problems by checking the sending IP against common blacklists
Vetting a newly assigned server or relay IP before putting it into production
Confirming a delisting request actually took effect after fixing an abuse issue

Common Mistakes

Treating a 'not listed' result here as a guarantee across every blacklist that exists; this checks four well-known ones, not the entire DNSBL ecosystem.
Requesting delisting before actually fixing the underlying spam or abuse issue, which usually just results in getting re-listed.

Tips

If Spamhaus results look unreliable here, check directly at their own website; their public lookup tool isn't affected by resolver-level rate limiting.

DNS Blacklist Checker