Enter the IP your mail is sent from and it is checked against 18 public email blacklists (DNSBLs), grouped by how much each one matters. Every list is verified with its own test entry first, so a list that refuses the lookup is never reported as clean. Free, no sign-up.
A DNS blacklist (DNSBL or RBL) is a list of IP addresses with a recent history of spam, compromise or abuse, published through DNS so mail servers can query it in milliseconds. A receiver reverses your IP (203.0.113.10 becomes 10.113.0.203), appends the list's zone and asks for an address: an answer in 127.0.0.0/8 means listed, no answer means clean.
Lists differ a lot. Some list single IPs that hit spam traps and expire them within a day; others list whole networks. A listing on a major list causes real delivery problems; one on a small or network-level list rarely does. Some major lists, Spamhaus among them, refuse queries from public DNS resolvers. Those are marked Unavailable here with a link to check them directly, instead of being shown as clean.
Spamhaus, SpamCop, Barracuda, UCEPROTECT Level 1, PSBL and Mailspike. Widely used; a listing hurts.
Trap-fed and reputation lists used by some filters as one signal among many.
UCEPROTECT Levels 2 and 3 list whole ranges. Shared-hosting neighbours can land you there.
Each list's test entry (127.0.0.2) must answer listed and its never-listed entry clean before the result is trusted.
Find out why first: follow the list's lookup link for the listing reason. Fix the cause (a compromised account, a form that lets bots send mail, a purchased list), then request removal. Many lists delist automatically within 24 hours to 7 days once the problem stops.
Not always. On shared IPs (most email platforms' default) a neighbour's behaviour can list the IP, and network-level lists penalize whole providers. If you send through a platform, tell their support; they manage their IP reputation.
Almost no blacklist publishes IPv6 data, and most bulk mail is still sent over IPv4.
Spamhaus blocks queries that arrive through public DNS resolvers, which is how this browser-based check works. Use the link to check it on Spamhaus's own site.
Point an MCP client (Claude, Cursor, any agent) at https://powerline.ai/mcp and call check_blacklists, or hit the JSON endpoint directly. No key, no account.
curl "https://powerline.ai/api/tools/blacklist?target=203.0.113.10"
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SPF, DKIM, DMARC, MX, MTA-STS, TLS-RPT, BIMI and blacklists in one run, scored out of 100 with a fix list.
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