Blacklist Check
Check whether a mail server is listed on the major DNS-based blacklists used to filter spam.
What is a DNS-based blacklist?
A DNSBL (DNS-based blacklist), also called an RBL, is a published list of IP addresses associated with spam, malware, open relays, or compromised hosts. Receiving mail servers query these lists during the SMTP conversation: they take the connecting IP, reverse it, append the blacklist's domain (e.g. zen.spamhaus.org), and look up that name. If the lookup returns an answer, the IP is listed — and the mail server can reject, defer, or score the message accordingly.
Being on a major blacklist — especially Spamhaus ZEN — is one of the most common reasons legitimate mail gets rejected or routed to spam.
What this tool checks
- Spamhaus ZEN — combines the SBL, XBL, and PBL. The single most widely-honoured blacklist; most mailbox providers use it.
- Barracuda BRBL — Barracuda's free reputation list, widely used by their appliances.
- SpamCop — built from user-reported spam complaints.
- SORBS — long-running list of dynamic ranges, open relays, and known spam sources.
- PSBL (Passive Spam Block List) — built from passive spam traps; lower noise.
For each IP we also fetch the PTR record (reverse DNS) — its absence or mismatch with the HELO/EHLO hostname is a separate strike against deliverability that most receivers also check.
What to do if you're listed
- Identify why. Click the "Delist" link on the affected blacklist to see what triggered the listing — often it's spam complaints, a compromised account, or a relay misconfiguration.
- Fix the cause first. Don't just request delisting before the underlying issue is fixed — most lists will re-list and may add penalty time.
- Request removal. Each list has its own removal procedure. Spamhaus and Barracuda are usually quick (hours to a day). SORBS can take longer.
- Verify after. Re-run this check a few hours later to confirm. Some receivers cache listings for up to a day.
Domain vs IP
DNSBLs list IP addresses, not domains. When you enter a domain here, we look up its MX records, resolve each MX server's IP, and check those IPs. That tells you the reputation of the servers your mail comes from. If you want to check a specific IP — for instance a self-hosted SMTP server or a sender you're tracking down — enter the IP directly.
Frequently Asked Questions
I just got listed — why?
The most common causes are: a compromised account or device sending spam, an open relay or misconfigured SMTP authentication, a marketing send that triggered many complaints, or an old IP that inherited a previous tenant's reputation. The blacklist's listing page usually says which one applies.
How long does delisting take?
Spamhaus and Barracuda are typically a few hours after you fix the cause and submit a removal request. SpamCop expires listings automatically after ~24 hours of clean traffic. SORBS can take days. Most large mailbox providers will check the lists in real time, so delisting is reflected quickly on their side once it propagates.
Why does this only check 5 blacklists when there are dozens?
The 5 we query are the ones most major mailbox providers actually use. Dozens of "URI" and niche blacklists exist, but listings on them rarely affect real-world deliverability. We chose breadth over depth so the check stays fast (<500ms) and the results stay actionable.
What is reverse DNS and why does it matter?
A PTR record maps an IP back to a hostname — the inverse of an A record. Receiving mail servers expect SMTP-sending IPs to have a PTR record that matches (or at least resolves cleanly) with the hostname presented in HELO/EHLO. Missing or mismatched PTR is itself a deliverability red flag, independent of any blacklist.
Can a domain itself be on a blacklist?
There are domain-based blacklists (e.g. SURBL, Spamhaus DBL for URLs found in spam bodies), but they're a separate category from the IP DNSBLs this tool checks. The most common deliverability failures come from IP listings, which is what we focus on here.
Why is my IP listed on SORBS but nowhere else?
SORBS is the most aggressive of the five — it lists entire dynamic IP ranges (DUL) and is quick to list compromised hosts. If you're only listed there, it's often because your IP is in a "consumer" range that SORBS has flagged en bloc, or there was a single past incident. Some receivers ignore SORBS for this reason.