What is a blocklist?
Also called: blacklist, DNSBL, RBL
A blocklist is a published list of IP addresses or domains known for sending spam, which receiving mail servers query in real time to decide whether to refuse a message. Major lists like Spamhaus are consulted by a large share of the internet's mail servers, so a listing can stop delivery almost entirely.
Listings usually come from one of three things: a spike in hard bounces, complaints from recipients, or mail hitting spam traps. A shared IP can also get you listed for someone else's behaviour.
Each list runs its own delisting process. Some expire automatically after a quiet period; others require a request and evidence that the cause is fixed. Requesting delisting without fixing the cause simply produces a second listing.
Checking your own domain against the major lists takes seconds and is worth doing before diagnosing anything else, because a listing explains a delivery collapse that otherwise looks inexplicable.
Last reviewed 2026-09-11