The DNSBL landscape in 2026

DNS-based blacklists (DNSBLs, sometimes RBLs) are reputation lists queried over DNS. There are dozens, and they aren't all equal. Here's the working list we maintain.

Tier 1 — fix immediately

Spamhaus ZEN. A combined list of SBL, CSS, XBL and PBL. Used by virtually every large mail receiver. Listing here translates to "your mail will be rejected by serious senders." The delisting process is well-documented and usually fast for false positives.

Tier 2 — fix if possible

• Barracuda Reputation Block List. Used by mid-market corporate filters running Barracuda hardware.
• SpamCop. Reactive; driven by user complaints. Listings expire automatically once complaints stop.
• SORBS aggregate. A large family of sub-lists. Listings here annoy a long tail of small receivers.

Tier 3 — informational

WillItInbox checks several more (UCEPROTECT, NiX, JIPPG, ivmSIP, S5H, PSBL) because they occasionally surface real problems, but a single listing on one of these alone shouldn't trigger action. Pattern matters: if you're listed on five Tier 3 lists at once, something is wrong with the IP.

What we deliberately don't check

We skip a number of well-known but problematic lists: ones with no public delisting policy, ones that haven't been updated in years, and ones that list entire /16 ranges over a single complaint. Adding noise to the report doesn't help.

How to handle a listing

1. Confirm the listing on the list operator's lookup page (don't trust a third-party tool alone).
2. Identify the cause. Compromised mailbox, misconfigured forwarder, and outbound spam from a customer are the top three.
3. Fix the cause. A delisting that happens before the cause is fixed will re-list within hours.
4. Submit a delisting request through the operator's official form.
5. Wait. Most legitimate delistings are processed in hours; some take days.

If you only do one thing

Monitor Spamhaus ZEN proactively. Set up a weekly check (cron + dig) on your sending IPs. Knowing within hours of a listing is the difference between a minor blip and a multi-day outage.