Blacklist check
See whether an IP or domain is listed on major spam blacklists.
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What is a DNS blacklist?
A DNS blacklist, or DNSBL, is a published list of IP addresses and domains known for sending spam or hosting abuse, which mail servers and filters consult before accepting a message. If your address appears on a widely used list, your email can be rejected or sent to spam even when everything else is set up correctly. This tool checks an IP address or domain against a set of major blacklists and shows which, if any, have it listed, along with a summary count. Enter an IP or domain to check it. A listing often follows a compromised server, a shared address with a bad neighbour, or a past spam incident. Use it to explain sudden delivery failures, to verify a new server address is clean before you send, or to confirm a delisting request has taken effect.
Key facts
- A DNSBL (DNS-based blocklist), also called an RBL, is a list of IP addresses or domains published over DNS that receiving mail servers query to decide whether to accept, reject or spam-folder a message.
- A mail server checks an IP by reversing it, prepending it to the blocklist zone (for example 2.0.0.127.zen.spamhaus.org) and doing an A-record lookup, where a 127.0.0.x answer means listed and NXDOMAIN means clean.
- Spamhaus ZEN is the most consequential blocklist because it combines the SBL, XBL, CSS and PBL sub-lists and is queried by a large share of receivers.
- An IP blacklist judges the sending server's IP address, while a domain blacklist such as Spamhaus DBL or the SURBL/URIBL lists judges the domains and URLs inside the message body.
- On shared hosting many sites share one public IP, so if a neighbour sends spam or gets compromised the whole IP can be listed and every tenant's mail is affected.
What a DNSBL (RBL) actually is
A DNSBL (DNS-based blocklist), also called an RBL (real-time blocklist), is a list of IP addresses or domains published over DNS that receiving mail servers query to decide whether to accept, reject or spam-folder an incoming message. When a server gets a connection from an IP, it reverses the IP, prepends it to the blocklist's zone (for example 2.0.0.127.zen.spamhaus.org) and does an A-record lookup. A returned 127.0.0.x answer means listed; NXDOMAIN means clean.
This tool runs those same DNS queries for you against the major public lists at once, so instead of manually resolving each zone you get a single pass/fail per list. A listing does not physically block your server; it is a reputation signal that thousands of receivers consult on every SMTP connection.
The lists that matter, and why
Not all blocklists carry equal weight. Spamhaus ZEN is the one to watch: it is a combined zone bundling the SBL (spam sources), the CSS (snowshoe and low-reputation senders), the XBL (compromised or exploited machines, largely mirroring the CBL), and the PBL (IP ranges the operator says should never send mail directly, such as residential and dynamic pools). A hit on any of these shows up under ZEN, and a large share of the world's inbound mail is filtered against it.
Barracuda Reputation Block List (BRBL) and SORBS are also widely consulted, and receivers like Microsoft and Google run their own internal reputation systems that no public tool can query. Treat a Spamhaus ZEN or Barracuda listing as high severity, and a hit on a small or aggressive niche list as lower priority - some receivers use it, many ignore it.
IP blacklists vs domain blacklists
These are two different reputation layers and both can sink a campaign. An IP blacklist (the DNSBLs above) judges the sending mail server's IP address. A domain blacklist judges the domains and URLs found inside the message body and headers - Spamhaus DBL and the SURBL/URIBL lists work this way, catching the domains spammers link to even when they rotate sending IPs.
So your sending IP can be spotless while a link domain in your newsletter is flagged, or your IP can be listed while your domains are fine. Check both: if the IP is clean but mail still fails, inspect the domains and shortened URLs in your content, and confirm your From-domain authentication is aligned.
Why an IP gets listed, including shared hosting
Common triggers are outbound spam from a compromised account or web app, a misconfigured open relay or open proxy, a server infected and joined to a botnet, sending to spam-trap addresses, sudden unusual volume, or simply a weak sending history with no SPF/DKIM/DMARC to vouch for you. Poor sending reputation accumulates over time, so warming a new IP slowly matters.
On shared hosting the IP is not yours alone. If a neighbour on the same address sends spam or gets compromised, the shared IP can be listed and every tenant's mail suffers, even senders who did nothing wrong. This is why serious senders use a dedicated IP or a reputable relay. If you inherited a fresh server, run this check first - the previous occupant's history may already be attached to the address.
Glossary
- DNSBL / RBL
- A DNS-based blocklist (also called a real-time blocklist) is a list of IP addresses or domains published over DNS that receiving mail servers query to decide whether to accept, reject or spam-folder a message. A returned 127.0.0.x answer means listed, while NXDOMAIN means clean.
- Spamhaus ZEN
- The most consequential blocklist, combining the SBL (spam sources), XBL (compromised machines), CSS (low-reputation senders) and PBL (IP ranges that should not send mail directly) into one queried zone. A large share of the world's inbound mail is filtered against it, so a hit is high severity.
- Domain blacklist
- A reputation list such as Spamhaus DBL or the SURBL/URIBL lists that judges the domains and URLs inside a message body rather than the sending IP. A sending IP can be clean while a link domain in the content is flagged, so both layers must be checked.
- Sender reputation
- The accumulated trust a sending IP or domain earns from its history, which receivers weigh on every SMTP connection. Outbound spam, hitting spam traps, sudden volume spikes and missing SPF/DKIM/DMARC all degrade it, which is why new IPs are warmed up slowly.
- Delisting
- The process of removing an IP or domain from a blocklist, done on the specific list's own removal page after the underlying abuse is fixed. Requesting removal without stopping the source usually results in being re-listed quickly.
- Shared IP listing
- A blocklisting that affects every tenant on a shared-hosting IP when one site on that address sends spam or is compromised. A dedicated IP or a reputable sending relay isolates a sender's reputation from neighbours on the same address.
Questions, answered.
What is a DNSBL and how does it differ from an RBL?+
A DNSBL (DNS-based blocklist) is a list of IPs or domains published over DNS that mail servers query to filter incoming mail. RBL (real-time blocklist) is an older term for the same concept - the phrases are used interchangeably. A receiving server resolves your IP against the list's DNS zone; a 127.0.0.x answer means you are listed.
Which blacklists should I care about most?+
Spamhaus ZEN is the most consequential because it combines the SBL, XBL, CSS and PBL and is queried by a huge share of receivers. Barracuda (BRBL) and SORBS are also widely used. A hit on ZEN or Barracuda is high severity; a hit on a small niche list matters far less because most receivers do not consult it.
Why did my IP get blacklisted?+
Typical causes are outbound spam from a hacked account or web app, an open relay or open proxy, a botnet infection, hitting spam-trap addresses, an abrupt spike in volume, or a weak sending history with no email authentication. Listings reflect the address's behaviour, so identify and stop the source before asking for removal.
Can I be blacklisted because of another customer on shared hosting?+
Yes. On shared hosting many sites share one public IP, so if a neighbour sends spam or gets compromised the whole IP can be listed and every tenant's mail is affected, even innocent ones. A dedicated IP or a reputable sending relay isolates your reputation from neighbours.
How do I get removed from a blacklist?+
Fix the underlying problem first - secure compromised accounts, close any open relay, and verify SPF, DKIM and DMARC pass. Then submit a delisting request on the specific blocklist's own removal page (Spamhaus, Barracuda and SORBS each have their own). If you request removal without fixing the cause, you will usually be re-listed quickly.
What is the difference between an IP blacklist and a domain blacklist?+
An IP blacklist (a DNSBL) judges the sending server's IP address. A domain blacklist such as Spamhaus DBL or the SURBL/URIBL lists judges the domains and URLs inside the message. Your IP can be clean while a link domain in your content is flagged, or vice versa, so check both layers when mail fails.
Does being on a blacklist always mean my email is blocked?+
No. Each receiver chooses which lists to use and how to weight them - some reject outright, some only add spam score, and many ignore minor lists entirely. A Spamhaus ZEN listing usually causes real delivery failures, while a hit on an obscure list may have almost no effect. Still, treat any listing as worth resolving.
How long does a blacklist listing last?+
It varies by list. Some, like Spamhaus PBL and XBL entries, clear automatically once the bad behaviour stops or after you self-remove; others persist until you request delisting and pass review. Nothing clears if the abuse continues, so remediate the source, submit removal where required, then re-check before sending again.
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