How DNSBLs work
A DNS blocklist publishes listings as DNS records. To check 203.0.113.5 against a list, a mail server reverses the octets and looks up 5.113.0.203.list.example.org. If a record comes back, the address is listed; if the lookup returns nothing, it is not.
Because it is just DNS, the check is fast and cheap, which is why virtually every mail server does it on every inbound connection. It is also why being listed has immediate consequences -- your mail starts bouncing straight away.
Why results here are often inconclusive
Major blocklists, Spamhaus in particular, refuse queries that arrive from public resolvers and datacenter ranges. They are funded by commercial subscriptions and block bulk querying to protect that model.
This tool runs in a datacenter, so those refusals apply to it. It probes each list with a known-listed test record first, and when a list does not answer properly it reports the result as inconclusive rather than reporting clean. A result you cannot verify is worse than no result, because it invites false confidence.
What to do if you are genuinely listed
Find the cause before requesting removal, because delisting without fixing the source leads straight back to a listing. The usual causes are a compromised mailbox sending spam, a misconfigured server acting as an open relay, an infected machine on your network, or a marketing campaign sent to a purchased list.
Once fixed, each blocklist has its own removal process. Most offer self-service delisting that works quickly for a first offence. Repeat listings take longer and some lists refuse automated removal entirely.
Reputation is broader than blocklists
Large providers such as Gmail and Outlook run their own internal reputation systems that no public tool can query. You can be absent from every public blocklist and still have mail filtered to spam by Gmail based on engagement signals and complaint rates.
Consistent authentication -- SPF, DKIM and DMARC all aligned -- matters as much as blocklist status. The Mail Health Checker covers those.
Frequently asked questions
Why does it say inconclusive rather than clean?
Because the blocklist declined to answer a query from this server's network. Rather than report a clean result it could not confirm, the tool tells you the check did not complete.
How long does delisting take?
Self-service removal from most lists takes effect within hours. Reputation with the large mailbox providers recovers more slowly, over days or weeks of clean sending.
Can I check a domain instead of an address?
Enter a domain and its A record is resolved and checked. Note that the address serving your website is often not the one sending your mail -- check the sending address for delivery problems.
Do these lists only apply to IPv4?
The lists checked here support IPv4 only. IPv6 blocklisting exists but is far less standardised, since the address space is too large for listing individual addresses to be practical.