A receiver bouncing your mail with “your IP is on a blocklist” is one of those moments where five seconds of objective information saves an hour of arguing. There are dozens of DNS-based blackhole lists in the wild; some matter a great deal to inbox placement, others nobody has used since 2014. The PortJar IP Blacklist Check queries seven of the lists that still matter, in parallel, and tells you exactly which (if any) of them are listing your address right now.
What the tool does
You give it an IPv4 address and it queries seven well-known DNSBLs — Spamhaus zen, SpamCop, Barracuda, and others in that tier — in parallel. For each list, it reports whether the IP is listed, and if so, includes the listing reason when the list publishes one. The queries run against the operators’ public DNS-based interfaces, which is the same mechanism mail servers use when they evaluate inbound mail in real time.
The seven lists are the ones with measurable impact on inbox placement at major receivers. The tool deliberately does not check the long tail of obscure or dead lists — if you’ve seen a “blocklist scanner” that lists 80 zones and 75 of them are unmaintained, you’ve seen why that approach is unhelpful. A clean result here means the IP is clean by the standards receivers actually apply.
How to use it
Open portjar.com/tools/rbl-check, paste the IP you want to check, and read the parallel results. Run it against the actual sending IP — the one that appears in the Received: header of bounced mail, not the office IP or the website IP. If your mail flow includes a smart host or a transactional service, the sending IP is the smart host’s, not yours.
If you control multiple sending IPs (a cluster, multiple regions, a fallback), check each one separately. A listing on one IP and a clean result on three others means one host in the rotation is compromised, misconfigured, or sharing a /24 with a noisy neighbour.
When you’d reach for it
- A receiver bounced with a blocklist message. Before you reply to the customer, confirm or refute the listing. If you’re listed on one of the seven that matter, you have the next step. If you’re not, the receiver’s bounce message is stale or wrong.
- You’re investigating a deliverability dip. Inbox placement has dropped over the last week and you want to rule out a fresh listing. A clean result rules it out; a hit explains the dip and tells you which list to talk to.
- You’re vetting a new IP allocation. A cloud provider has handed you a fresh IP for an outbound mail server. Before you point production traffic at it, check that the previous tenant didn’t leave it listed.
- Post-incident hygiene after a compromise. A WordPress site or a mail account was used to send spam. After the cleanup, check whether the IP picked up listings during the incident — that drives whether you also need to file removal requests.
- Proving a clean state to a contract or compliance reviewer. A copyable, dated report from a known tool is more useful than a screenshot of your own dashboard.
What to make of the output
A listing on Spamhaus zen is the one that almost always matters. Major receivers — Microsoft, Google, Yahoo — consult Spamhaus heavily, and a zen listing will hit your inbox placement within hours. Treat it as urgent: identify the source of the abuse signal, fix it, then request delisting through Spamhaus’s portal once you can demonstrate the cause is resolved.
A listing on SpamCop or Barracuda is meaningful but slower-moving. Some receivers use them, some don’t, and Barracuda in particular will often clear automatically once the source of complaints stops. Investigate the underlying cause anyway — a listing on either of those is rarely a false positive.
An error or timeout on one of the seven lists is not the same as a listing. Several RBL operators rate-limit DNS queries from busy resolvers, so a single query may time out without meaning anything about your IP. If a zone errors, retry from a different network or check via the operator’s own lookup page before treating the result as definitive.
A fully clean result with all seven lists answering “not listed” is the answer you want — but it is not a guarantee of inbox placement on its own. Authentication (SPF, DKIM, DMARC), content, and sender reputation still matter. The blocklist check tells you the IP itself isn’t actively flagged; it does not tell you the rest of the deliverability stack is correct.
For environments where outbound mail reputation needs to stay clean across many domains and tenants, Stack Harbor handles the deliverability discipline as part of cPanel and WHM management.