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RBL Blacklist Check

Check an IPv4 address against four major DNS-based blocklists in parallel — Spamhaus ZEN, SpamCop, Barracuda, and SORBS — and surface delisting hints.

About RBL Blacklist Check

The RBL Blacklist Check queries an IPv4 against four widely respected DNS-based blocklists in parallel: Spamhaus ZEN (the umbrella of SBL, XBL, and PBL), SpamCop's bl.spamcop.net, Barracuda's b.barracudacentral.org, and SORBS's dnsbl.sorbs.net. Each lookup reverses the IP octets and queries it as a hostname under the RBL zone — the standard DNSBL protocol. A 127.0.0.x A record means listed; NXDOMAIN means clean. The tool also pulls the TXT record from each list, usually containing a listing reason and a delisting URL. Use it when your sending IP starts producing 5.7.1 rejections, when an inbound mail server you operate is being refused at recipient gateways, or when you take over an IP from a previous tenant and want to confirm it isn't already poisoned. Queries run in parallel from our backend; the IP and listing results stay private to your session.

Why use RBL Blacklist Check

  • Four major lists in one query — Spamhaus ZEN, SpamCop, Barracuda, SORBS.
  • Parallel execution keeps the wall-clock under a couple of seconds.
  • Pulls the listing-reason TXT record so you know what triggered the listing.
  • Distinguishes ERROR (DNS failure) from CLEAN (NXDOMAIN) so you don't get false-clean results.
  • Pairs with SMTP Server Test and Email Header Analyzer for full deliverability triage.
  • No signup, no rate-limit beyond abuse protection — usable in an active incident.

How to use RBL Blacklist Check

  1. Enter the IPv4 address you want to check — for example, your sending MTA's public IP.
  2. Click Check blacklists to run four parallel DNSBL queries.
  3. Review the summary banner — green if clean across all four, red if listed on any.
  4. For each LISTED row, expand the TXT detail to see the listing reason and delisting URL.
  5. Open the linked delisting page on the listing site, follow their removal process, and re-test.
  6. If the IP is clean here but mail still bounces with 5.7.1, run the matching SMTP Server Test.

When to use RBL Blacklist Check

  • When recipients return 5.7.1 'listed in DNSBL' rejections.
  • After taking over a public IP from a previous tenant — confirm a clean reputation.
  • When inbound mail to a server you operate is bounced and you suspect the connecting IP.
  • Periodically as a scheduled deliverability audit on every sending IP you control.
  • When migrating a sending IP between cloud providers — verify it isn't pre-listed.
  • While building a sending-IP allowlist for a new mail platform launch.

Examples

Clean reputation

Input: 8.8.8.8

Output: Listed on 0 of 4 RBLs — clean across Spamhaus ZEN, SpamCop, Barracuda, SORBS

Listed on Spamhaus ZEN

Input: 203.0.113.42

Output: Listed on 1 of 4 RBLs · Spamhaus ZEN: LISTED · TXT: 'See https://www.spamhaus.org/sbl/query/SBL12345'

Multi-list listing (compromised host)

Input: 198.51.100.7

Output: Listed on 3 of 4 RBLs · Spamhaus ZEN, SpamCop, Barracuda all flagged this IP

Tips

  • Spamhaus ZEN is the heaviest-hitting of the four — a ZEN listing usually means broad rejection.
  • SORBS often takes 24-48 hours to expire automatic listings even after the abuse stops.
  • Barracuda requires a manual delisting request via their website for most listings.
  • If you're listed on PBL (a sub-list of ZEN), you don't need to delist — just send through an authenticated MTA.
  • Some DNSBLs only list dynamic-IP ranges (PBL); your VPS may be pre-listed without you doing anything wrong.

Frequently Asked Questions

Is the queried IP sent to any third party?▾
Each DNSBL query is, by definition, a DNS query to that DNSBL's resolver — Spamhaus, SpamCop, Barracuda, SORBS see your query because that's how DNSBL works. We don't store the IP or share it with anyone else.
How does this compare to mxtoolbox's blacklist check?▾
We hit four high-signal lists in parallel rather than 70+ low-signal ones. The four we use account for the bulk of real rejections; the long-tail mxtoolbox queries return many false positives. Ours is also free with no signup.
What does LISTED actually mean for my email deliverability?▾
It means at least one major receiver class is configured to refuse mail from your IP. The blast radius depends on the list — Spamhaus ZEN affects the most receivers; a SORBS listing alone is usually less impactful.
Can this detect a misconfigured MTA?▾
It detects the symptom (a listing). To diagnose the cause you usually need to pair this with Email Header Analyzer, SMTP Server Test, and your MTA's own logs.
Why does the tool say IPv4 only?▾
Most DNSBLs predate IPv6 and don't list AAAA addresses. Querying an IPv6 against an IPv4-only zone returns a guaranteed clean result and would mislead you. We restrict input to IPv4 to avoid false reassurance.
What does 'soft fail' mean here?▾
DNSBLs themselves don't have a soft/hard fail concept — that's an SPF term. A DNSBL query is binary: listed (any 127.0.0.x answer) or clean (NXDOMAIN). What downstream MTAs do with the listing is configurable per receiver.
How do I get delisted from Spamhaus or SpamCop?▾
Each list publishes a delisting URL in the TXT record we surface. Spamhaus typically requires you to fix the abuse source first; SpamCop expires listings automatically after 24 hours of clean traffic; Barracuda requires a manual request.
Can a clean RBL result still mean my mail will bounce?▾
Yes. RBL listing is just one of many deliverability signals. SPF/DKIM/DMARC failures, content filtering, sender-domain reputation, and recipient-specific rules all separately reject mail.

Explore the category

SMTP Server Test Connect to any SMTP host on port 25 (or a custom port), capture the banner, send EHLO, and verify whether STARTTLS is advertised. Email Header Analyzer Summarize Received chains and auth hints from pasted headers. MX Record Lookup Look up the Mail Exchange (MX) records for any domain to see which mail servers handle its incoming email. SPF Record Checker Look up and parse a domain's SPF DNS record. Validates mechanisms, counts DNS lookups against the RFC 7208 ten-lookup limit, and flags soft/hard fail qualifiers. DMARC Record Checker Look up the DMARC TXT record at _dmarc.<domain>, parse every tag (p, sp, pct, rua, ruf, adkim, aspf), and flag missing aggregate report addresses or partial enforcement. Reverse DNS Lookup Perform a reverse DNS lookup (PTR record query) to find the hostname associated with any IP address.
How DNS Works: A Step-by-Step Journey from Domain to IP You type a domain and a page loads — but between those two events is a surprisingly elegant distributed lookup system. Here is exactly what Feb 2026 · 9 min CIDR Notation Explained: How to Read /24, /16, and Why It Matters If a Slack message that contains '10.0.0.0/16' makes you nod and quietly Google 'what does /16 mean again', this is the post for you. Read / May 2026 · 10 min

Glossary

RBL
Realtime Blackhole List — a DNS-based list of IPs known for spam or abuse, queried by mail servers to refuse connections.
DNSBL
DNS-based Blocklist — modern term for RBL. Same protocol: reverse-IP queries that return 127.0.0.x for listed addresses.
Spamhaus ZEN
Spamhaus's combined zone of SBL (server blocklist), XBL (exploits), and PBL (policy / dynamic IPs). The most widely used DNSBL globally.
PBL
Policy Block List — Spamhaus's list of dynamic-IP ranges that should not send direct-to-MX mail. A PBL listing is normal for residential IPs.
Listing TXT record
Most DNSBLs publish a TXT alongside the A record explaining why the IP is listed and how to request removal.
5.7.1 rejection
An SMTP permanent rejection code typically returned when an MTA refuses a connection due to a DNSBL listing or sender-policy failure.