What is an email blocklist?
Also called: blacklist, DNSBL, RBL
A blocklist is a published list of IP addresses or domains associated with spam, which receiving mail servers query in real time to decide whether to accept a message. Most are served over DNS, so they are called DNSBLs. A listing on a widely used list such as Spamhaus can stop delivery to many providers at once.
The DNS method is described in RFC 5782: the receiving server reverses the connecting IP and looks it up under the list's zone. An answer means listed.
Listings usually follow a spike in hard bounces, complaints, or mail hitting spam traps. On a shared IP you can be listed for someone else's behaviour.
Each list runs its own removal process. Some listings expire after a quiet period; others need a request and evidence that the cause is fixed. Asking for removal without fixing the cause usually produces a second listing.
Example
5.113.0.203.zen.spamhaus.org. A reply in 127.0.0.x means listed.Go deeper
Sources
By SimpleVerifier Team · Last reviewed 2026-10-13 · All 50 glossary terms