IP Address / Domain Blacklist (DNSBL) Checker
Enter an IP address or domain to check it against major DNSBLs (blacklists) such as Spamhaus, SORBS, Barracuda, and SpamCop all at once to see if it's been flagged as a spam source.
Checking whether an address is on a blocklist
Enter an IP address or a domain and this tool queries the principal DNSBLs at once, among them Spamhaus, SORBS, Barracuda and SpamCop, and reports whether it is listed. It is the first place to look when investigating why mail is not arriving.
**The commonest misunderstanding here is to read a listing as proof that spam was sent.** Addresses belonging to cloud providers and shared hosting are frequently **still listed on account of whoever held the address before**, so one can be caught up in it having done nothing at all. **The second point is that the lists differ entirely in how they are run and in how much they matter.** One consulted by a great many receiving systems, as Spamhaus is, should not be treated on a par with one almost nobody looks at. **And the cause must be put right before any delisting is requested.** Delisting while the server remains an open relay, or while a machine inside the network is infected, simply leads to being listed again at once.
How to use it
- Enter an IP address or a domain The usual thing is to enter the address of your own mail server.
- Confirm whether it is listed **The lists on which it appears are set out together.**
- Attend to the consequential lists first **Begin with those consulted widely, such as Spamhaus.**
- Put the cause right before requesting delisting **While the cause remains, delisting is undone almost immediately.**
Tips for getting more out of it
- If your emails aren't reaching recipients or keep landing in spam folders, start by checking here whether your sending IP address is listed on a blacklist.
- Enter a domain and this tool automatically resolves its A record (IPv4 address) first, then queries each blacklist — there's no need to look up the IP yourself in advance.
- If you send mail from a shared server or shared IP address, you can end up listed as collateral damage from another user's spam activity on the same IP.
- If a listing turns up, check the official site for that blacklist (linked from the results) for its delisting procedure. Most providers let you request removal for free.
- DNSBL data isn't always perfectly up to date, so it's best to check across multiple blacklists rather than basing a decision on a single result.
Where it comes in useful
Investigating why mail is not arriving
**It is the first thing to check when messages fail to reach people or are treated as spam.**
On taking on a new IP address
**You can establish before you begin whether it is already listed on account of its former holder.**
Keeping watch at intervals
A listing can occur without your noticing, which makes an occasional check worth the trouble.
Confirming that a delisting took
After making the request, you can see whether the entry really has gone.
DNSBL terms
- DNSBL
- **A blocklist queried through the DNS machinery.** Receiving servers use it in deciding whether to refuse a message.
- Spamhaus
- One of the most widely consulted lists. **A listing here carries great weight, so attend to it first.**
- Open relay
- **A server configured to forward mail from anyone at all.** It is abused for spam and becomes a cause of listing.
- Delisting request
- The procedure of asking the list's operator to remove an entry. **Undertaken before the cause is put right, it is promptly undone.**
- Shared address
- An arrangement in which several customers use the same IP address. **The conduct of one can see everyone listed.**
- Reputation
- How a sender is regarded. **Besides blocklists, a sudden rise in volume and the rate of complaints both bear on it.**
Frequently asked questions
Side Note — blacklists piggyback on the DNS system itself
DNSBL (DNS-based Blackhole List) is a technique devised by the anti-spam community in the late 1990s. Its defining feature is that instead of building a dedicated database server or API from scratch, it simply repurposed the DNS query infrastructure that was already deployed worldwide. The trick is deceptively simple: reverse the octets of an IP address and build a special hostname like "2.0.0.127.zen.spamhaus.org," then send that as an ordinary A record query. If the address is listed, you get back a response code like "127.0.0.2"; if not, you get NXDOMAIN.
This approach spread widely because every mail server of the era already had DNS lookup capability built in, so no additional software or protocol was ever required to adopt it. Today's major blacklist operators — Spamhaus, SORBS, Barracuda, and others — all still rely on this same simple DNS query format.
DNSBLs are not free of false positives, though. Dynamic IP ranges, or addresses that carry a history of spam sent by a previous occupant, can remain listed regardless of who's using the address now. That's exactly why most blacklist operators provide a delisting request process — flagging a listing and requesting removal promptly is one of the most effective ways to improve deliverability.