A DNS blocklist (DNSBL, often called a blacklist) is a published list of IP addresses or domains that an operator has seen sending spam or behaving abusively. Receiving mail servers query these lists during the SMTP conversation, and a hit can mean an instant rejection, a trip to the spam folder or a higher spam score.
This checker runs those same queries for you. It works entirely in your browser over DNS-over-HTTPS, so nothing you enter is logged or saved.
What this checker tests
You can enter either a domain or an IPv4 address.
- Domain: we resolve its MX hosts to IPs and also look up the apex A record, then test every IP.
- IPv4: we test that single address.
Each IP is checked against 15 public lists: Barracuda BRBL, SpamCop, PSBL, UCEPROTECT Level 1, Mailspike, GBUdb Truncate, DroneBL, S5H, Anonmails, SPFBL, 0spam, InterServer, Blocklist.de, SEM Black and SEM Backscatter.
The lookup itself is simple. The IP 203.0.113.25 is reversed to 25.113.0.203, the list zone is appended, and an A query is made:
25.113.0.203.b.barracudacentral.org A -> 127.0.0.2 (listed)
25.113.0.203.bl.spamcop.net A -> NXDOMAIN (not listed)Any answer in 127.0.0.0/8 means listed. No answer means clean on that list.
The major blocklists at a glance
Not all lists carry the same weight. This table covers the ones you are most likely to meet in a bounce message.
| List | What it targets | How delisting works |
|---|---|---|
| Spamhaus SBL | IPs and ranges tied to known spam operations | Fix the issue, then open a ticket via the Spamhaus lookup page |
| Spamhaus XBL (includes CBL) | Hijacked machines, botnets, open proxies | Self-service removal on the lookup page once the infection is cleaned |
| Spamhaus PBL | Dynamic and residential IPs that should not send direct mail | Self-removal if the IP is a real mail server with proper PTR |
| Spamhaus DBL | Domains found in spam message bodies and headers | Request via the lookup page after fixing the source |
| Barracuda BRBL | IPs sending spam to Barracuda-protected networks | Free removal request form, usually handled within a day |
| SpamCop | IPs reported by SpamCop users | Automatic expiry roughly 24 hours after reports stop |
| UCEPROTECT L1 | Single IPs hitting their spam traps | Automatic after seven days clean; paid express removal is optional |
| PSBL | IPs that hit its spam traps | Instant self-removal on the PSBL site |
| Mailspike | IPs scored on reputation over time | Clears automatically as reputation improves; request form available |
| DroneBL | Open proxies, bots and abuse sources, mostly non-mail | Removal request through its site |
| URIBL / SURBL | Domains that appear in links inside spam | Request via each list’s lookup page |
In bounce messages, a blocklist rejection often appears as a 550 5.7.1 or a 554 with the list name in the text.
How to read your results
For each IP you will see its source (MX host or apex A), the list name and a status of listed, clean or error.
Listed on one small list. Note it, check whether the list is still maintained and look at your recent sending. This rarely explains a broad deliverability drop on its own.
Listed on Barracuda, SpamCop or UCEPROTECT. Take it seriously. These are used by many business mail gateways, and mail to B2B recipients is likely being rejected.
Only the apex A record is listed. Your web server IP is flagged. If you do not send mail from it, the impact on inbox placement is small, though links to that domain can still score worse. Shared web hosting is the usual cause.
Error or timeout. The list did not answer in time. Rerun the check. A persistent error often means the list has shut down or restricted access.
Why IPs and domains get listed
Most listings come from a handful of causes.
- Spam traps. Old or purchased lists contain addresses that exist only to catch senders with poor hygiene. Hitting them is the fastest route onto UCEPROTECT and PSBL. See our list hygiene guide.
- Complaints. Mailing people who did not opt in drives SpamCop reports and feeds into Spamhaus data.
- Compromised accounts. A hacked mailbox, a WordPress contact form abused by bots or malware on a server can send thousands of messages in an hour.
- Shared infrastructure. On a shared IP, another customer’s behavior becomes your problem. UCEPROTECT Level 2 and 3 even list whole ranges and networks.
- Sudden volume spikes. A new IP that jumps from zero to 100,000 messages looks exactly like a spam run. Warm up gradually, as our IP warmup guide explains.
How to get delisted
Delisting only lasts if the cause is gone. Work in this order.
- Confirm the listing and read the reason. Each operator’s lookup page explains why an IP was added and sometimes when.
- Fix the source. Reset compromised passwords, close open relays, add CAPTCHA to forms, and remove unengaged and invalid addresses from your list.
- Request removal. Use the operator’s own form. Many lists expire automatically once the IP is quiet.
- Recheck. Run this checker again, plus Spamhaus directly, and watch bounce rates for the next few sends.
Our blacklist removal guide covers each major list’s process in more detail.
Staying off blocklists
Prevention is mostly permission and consistency. Use confirmed opt-in for new subscribers, suppress hard bounces immediately, sunset addresses that have not engaged in six months or more, and keep complaint rates below 0.1%. Publish SPF, DKIM and DMARC so receivers can tell your real mail from spoofed messages, and set a proper PTR record on every sending IP with the reverse DNS checker.
For a broader view that combines blocklists with authentication and DNS health, run the sender reputation checker.