Check any IP or domain against 12+ public DNSBLs in seconds.
Paste an IPv4 address or sending domain — we check it against Spamhaus, Barracuda, SpamCop, the SORBS family, UCEPROTECT, Mailspike, CBL, GBUdb, and PSBL in parallel. Listings tell you why mailbox providers are downranking your mail.
A DNSBL / RBL ("Real-time Blackhole List", "DNS Block List") is a public list of IPs or domains known to send spam, host phishing, or be part of compromised infrastructure. Each list publishes its data over DNS so mailbox providers can query it for every incoming connection without rate-limiting concerns.
When mail from your IP arrives, the receiver does a reverse-DNS lookup against each list it subscribes to. A listing on Spamhaus ZEN is the single biggest signal — most mailbox providers will outright reject mail from a ZEN-listed IP. Listings on smaller lists usually downrank rather than reject.
Common reasons to get listed: shared-IP host got compromised, your domain got spoofed and the spoof traffic looks like it came from you, your warmup ramp was too aggressive, your list hygiene let spam-traps through, or a previous tenant of the IP burned the reputation. Each list publishes its own removal instructions; the ones run by reputable organisations process delisting in 24–48h.
A clean check now doesn't guarantee a clean check tomorrow. The pro version of this tool — the Blacklist Monitoring add-on inside SieveGuard — checks 90+ lists daily on every IP and domain you onboard and alerts you the moment something changes.
Spamhaus ZEN, Barracuda BRBL, SpamCop SCBL, SORBS (dnsbl + spam), CBL/Abuseat, PSBL, UCEPROTECT (L1/L2/L3), Mailspike, GBUdb Truncate. Twelve lists in total, covering ~95% of what mailbox providers actually use.
L2 lists /24 networks — your specific IP isn't individually flagged, but a neighbour on the same /24 is. UCEPROTECT is the only major list that does network-level listings, and most mailbox providers don't consult L2 / L3 directly. Less urgent than a SBL hit.
Each list publishes its own delisting process. Spamhaus has a self-service portal for ZEN; Barracuda has a delisting form; SpamCop expects the issue (compromise / list hygiene) to be resolved before the listing auto-expires. Links are on each list's site.
Possible — not every mailbox provider checks every list. Major providers check Spamhaus heavily and SpamCop / Barracuda moderately; small/regional lists are usually consulted by smaller mail servers only.
Not via this tool — DNSBL lookups are per-IP. The Blacklist Monitoring add-on inside SieveGuard supports CIDR-range monitoring for tenants who manage IP allocations.
SieveGuard's Blacklist Monitoring add-on runs this check daily on every sending IP and domain you onboard, against 90+ lists, with alerts the moment a new listing appears. Catch drift in hours, not weeks.