Why does a listing say unknown instead of clean?

Some lists, Spamhaus in particular, refuse queries that arrive through public resolvers or over a free quota, and return an error code instead of an answer. Reporting that as clean would be a lie, so it is shown as unknown.

How do I get delisted?

Fix the cause first (compromised account, open relay, missing PTR), then use the list's own removal page, linked in the results. Most lists remove automatically after a period without new spam.

Which blacklists matter most?

Spamhaus ZEN is used by the largest share of receivers. SpamCop and Barracuda are common in gateway products. Domain lists such as SURBL and URIBL affect links inside messages more than the sending IP.

Blacklist checker

A DNSBL (DNS-based blocklist) is a published list of IP addresses or domains reported for sending spam, which receivers query before accepting a connection. This tool tests a sending IP or domain against the blocklists receivers actually query, and gives the delisting link for anything that hits.

What this tool checks

For an IP address, the octets are reversed and prefixed to each blocklist zone (so 203.0.113.5 becomes 5.113.0.203.zen.spamhaus.org) and the zone is queried for an A record. An answer means listed; a name-not-found means clean. For a domain, the domain-based lists (Spamhaus DBL, SURBL, URIBL) are queried directly, and the IP lists are run against the address of the domain's primary MX.

Reading the codes

Lists encode the reason in the returned address. Spamhaus 127.0.0.2 through 127.0.0.4 are spam sources, 127.0.0.10 and .11 are policy listings for dynamic address ranges. Anything in 127.255.255.0/24 is not a listing but a refusal to answer, which this tool shows as unknown.

Blocklists at the handshake

A DNSBL hit is available the moment a connection opens, before any SMTP command. Spamjadoo queries the lists you configure at that point and answers with a 550 if the policy says so, which is why listed senders never consume bandwidth, storage or scanning time on a Spamjadoo-protected domain.

Questions people ask

Why does a listing say unknown instead of clean?: Some lists, Spamhaus in particular, refuse queries that arrive through public resolvers or over a free quota, and return an error code instead of an answer. Reporting that as clean would be a lie, so it is shown as unknown.
How do I get delisted?: Fix the cause first (compromised account, open relay, missing PTR), then use the list's own removal page, linked in the results. Most lists remove automatically after a period without new spam.
Which blacklists matter most?: Spamhaus ZEN is used by the largest share of receivers. SpamCop and Barracuda are common in gateway products. Domain lists such as SURBL and URIBL affect links inside messages more than the sending IP.