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Spamjadoo

What is a DNSBL (DNS blocklist)?

A DNSBL (DNS-based blocklist, also called an RBL) is a list of IP addresses or domain names associated with spam, published as a DNS zone. A mail server checks a connecting IP by reversing its octets, appending the zone name, and asking for an A record: an answer means listed. Because the check is a single DNS lookup it can be applied the instant a connection opens.

The query

To test 203.0.113.5 against Spamhaus ZEN, the server asks DNS for:

5.113.0.203.zen.spamhaus.org.  A ?

No record means clean. A record in 127.0.0.0/8 means listed, and the last octet encodes why: for Spamhaus, .2 through .4 are spam sources, .10 and .11 are policy listings for dynamic address ranges.

Lists that matter

  • Spamhaus ZEN is queried by the largest share of receivers. It requires paid access for commercial volumes and refuses queries via public resolvers.
  • SpamCop lists on user reports and delists automatically within 24 hours of the last report.
  • Barracuda and SORBS are common in gateway products.
  • SURBL and URIBL list domains found in spam links, not sending IPs.

The blacklist checker queries all of these.

Delisting

Fix the cause before requesting removal: a compromised account, an open relay, a missing PTR record, or an infected machine on the network. Then use the list’s own removal form. Requesting removal without fixing the cause gets an IP relisted quickly and, on some lists, for longer.

DNSBLs at the handshake

A DNSBL answer is available before the sender types a single SMTP command, which makes it the cheapest possible rejection. Spamjadoo queries the lists you configure at connection time and, per your policy, answers a listed source with a 550 that names the list, so a legitimate sender knows exactly where to go.

Last reviewed 10 September 2026 by Spamjadoo engineering.