What’s the difference between DNSBL and RBL?

RBL (Realtime Blackhole List) and DNSBL (DNS-based Block List) refer to the same technology. RBL was the original term, coined by Paul Vixie for the first such list. DNSBL is the more technically accurate description of how these systems work.

Both terms describe blocklists that are queried using DNS lookups. When a mail server wants to check if an IP is listed, it performs a DNS query against the blocklist's domain. A positive response means the IP is listed.

The term "blackhole" in RBL originally referred to routing spam into oblivion. As terminology evolved to be more inclusive, "blocklist" became preferred over "blacklist," making DNSBL the standard term.

In practice, the terms are interchangeable. You may encounter either in documentation and discussion.

Different names, same harbor registry. Both describe lists queried through DNS to identify problematic senders.