What the blacklist checker does
Enter an IP address or a domain. A domain is first resolved to its IP address, which is then checked against 30 DNS-based blocklists (DNSBLs) at once, including Spamhaus ZEN, SpamCop, Barracuda, SORBS, UCEPROTECT and Mailspike.
Mail servers query these lists for every incoming connection. If your sending IP is listed on a list a receiver uses, your mail is rejected or sent to spam no matter how well your SPF, DKIM and DMARC are set up.
Reading the result
Each list returns listed, not listed or an error. An error is not a listing: some lists, such as Spamhaus, refuse queries that come from large public resolvers, and others rate-limit them. Only a listed answer means the IP is on that list.
Being listed on one minor list rarely matters. A listing on Spamhaus, Barracuda or SpamCop does, because major providers rely on them.
Frequently asked questions
Why is my IP on a blacklist?
Usually because spam was sent from it: a compromised account or server, an open relay, or a previous owner of the IP. On shared or residential ranges, some lists include whole blocks of addresses regardless of what you send.
How do I get removed from a blacklist?
Fix the cause first, then use the removal form of the list that names you. Spamhaus, Barracuda and most others have one, and many lists also delist automatically after a period without new spam.
Why do some lists show an error?
Several DNSBLs block or throttle queries from public resolvers and high-volume clients. An error means the list did not give an answer, not that you are listed.