DNS Tools
MAIL REPUTATION

Email Blacklist Check

Enter a domain or mail-server IP. The tool queries only the blocklist zones its operator has configured and licensed. If none are enabled it says so and does not report the target as clean.

Run DNS blocklist check

No sign-up required. Public hosts and addresses only; results show what was observed, nothing is simulated.

What a DNS blocklist is

A DNS blocklist (DNSBL) is a list published through DNS. To check an IPv4 address, the client reverses its octets and appends the list's zone name, for example 10.2.0.192.zone.example.org, then asks for an A record. NXDOMAIN means the entry is not listed; an answer in the 127.0.0.0/8 range means it is listed, with the value usually coding the reason. Domain lists work the same way using the domain name as the key.

Mail servers consult these lists when deciding whether to accept, defer, reject or junk a message, so a listing of your sending IP or domain can quietly cost you deliverability.

Why a 'not configured' result is not a clean result

Many blocklist operators publish terms that restrict commercial or high-volume use, or forbid queries through public resolvers. This tool therefore queries nothing by default. An operator must set the zone list and acknowledge the licence terms. If neither is done, the result is the 'not configured' state, with the reason and the setup steps. That state is not a pass: it means no blocklist was consulted, so nothing is known about the target.

Even when zones are configured, a 'not listed' answer covers those zones only. There is no single authoritative blocklist, and absence from the zones consulted does not establish a good reputation anywhere else.

How to read the result

Each configured zone appears in a table with its result, the return code and detail. LISTED means the zone returned a listing for the key. not listed means NXDOMAIN. UNAVAILABLE means the answer could not be used: DNS failed, the zone answered with no data, or it returned a refusal code in the 127.255.255.x range, which typically means the list refused the query because of the resolver used. If any zone is unavailable and none lists the target, the overall result is unknown rather than clean. The result is timestamped because listings change.

Common problems

  • Listed because of a compromised account or host. Spam sent from a hijacked mailbox is a frequent cause.
  • Listed because of a neighbour. Some lists cover whole address ranges, especially dynamic or shared IP space.
  • Refusal codes mistaken for listings. A query through a public resolver can return a refusal; read the code and detail.
  • List aimed at the wrong key. An IP list will not know a domain name, and a domain list will not know an IP, so it answers 'not listed'.
  • Stale delisting. Requesting removal before the cause is fixed leads to relisting.

How to respond to a listing safely

Prerequisites: access to outbound mail logs and the list operator's own lookup page, which is the authority on the reason for a listing. Operational risk: delisting requests do not fix the cause, and some operators lengthen the penalty on relisting. Rollback is not applicable because the tool changes nothing; the main risk is acting on an answer from a zone that refused the query.

  1. Open the operator's own lookup page for the zone and read the reason and any evidence.
  2. Identify the cause: compromised credentials, a misconfigured application, an open relay or a sender in your customer base.
  3. Fix and verify the cause, and check queue and logs for the period around the listing.
  4. Follow the operator's delisting procedure.
  5. Query the zone again and confirm NXDOMAIN, then send test mail to a recipient who uses that list.

Limits

Only operator-configured zones are queried, and answers depend on the resolver and on whether the zone accepts queries from it. The tool does not check spam-filter reputation systems that are not DNS-based, does not read mailbox-provider internal reputation, and does not estimate deliverability. A listing may also be shown only to some resolvers or networks. Always confirm on the operator's own site before taking action.

Frequently asked questions

The tool says no blocklist is configured. Am I clean?

No. That result means nothing was checked, so it says nothing about your IP or domain.

Why does the tool not check lots of lists automatically?

Many list operators restrict automated or commercial use and some block public resolvers. The server operator must configure lists and accept each licence.

What does a 127.255.255.x answer mean?

The list refused to answer, typically because of the resolver used. It is shown as unavailable, not as a listing.

How do I get delisted?

Follow the list operator's procedure after you have fixed the cause. Do not request delisting first.

Can a clean result guarantee delivery?

No. Delivery also depends on authentication, content, complaint rates and each provider's own policy.

Should I enter a domain or an IP?

Enter the sending mail server's IP for IP lists and the domain for domain lists. A list only answers for the kind of key it indexes.

Scope of this tool

  • Does nothing unless an operator configures zones and acknowledges their licence terms.

Written by DNS Tools editorial · Last updated 2026-10-09