DNSBL (DNS blocklist)
A DNSBL is a blocklist queried over DNS. Mail servers and websites use it to check whether an IP has a history of spam, abuse, or malware.
What a DNSBL is
A DNS-based blocklist, also written DNSBL or RBL, publishes a list of IP addresses through the DNS system. To ask whether 203.0.113.7 is listed, a client reverses the octets and queries 7.113.0.203.zen.spamhaus.org. An answer in the 127.0.0.x range means "listed", and the last number often encodes the reason: spam source, exploited machine, policy listing for a dynamic range, and so on. No answer means "not listed". The scheme is fast, cacheable, and works with any resolver, which is why it became the standard way to share reputation.
Each list has its own policy. Some list only confirmed spam senders; others list entire dynamic ranges on principle; a few list open proxies and botnet drones specifically.
Why it matters
Mail servers consult blocklists on every incoming connection, and a listing commonly means outright rejection. Beyond email, sign-up forms, comment systems, and fraud engines check the same lists to decide whether to challenge a visitor. For a proxy or VPN address the history belongs to whoever used it before you: a listed exit will see your messages bounce and your registrations fail even if you have done nothing.
Our anonymity score deducts 5 points per listing, capped at 20, and a separate 20 points if the address falls inside a Spamhaus DROP range, which marks hijacked or criminal networks rather than individual senders.
Which lists we check
The blacklist check queries eight lists: Spamhaus ZEN, SpamCop, Barracuda, SORBS, UCEPROTECT level 1, PSBL, s5h.net, and DroneBL. Results are cached for one hour. The lists are IPv4 only, because almost no blocklist publishes IPv6 entries. Each result links to the list's own site, where you can read the reason and request removal.
How to get delisted
Fix the cause first: a compromised machine, an open relay, or a misconfigured mailer will be relisted within days. Then follow the removal process on the list's site. Spamhaus PBL entries describe a dynamic range rather than your behavior and are removed by the ISP, not by you.
A common misconception
A listing on one aggressive list is not the same as a bad reputation. Look at which list fired and why; UCEPROTECT, for example, lists entire providers for the behavior of a few customers.