What’s the difference between DNSBL and RBL?
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You've probably seen both terms thrown around in deliverability discussions and wondered if someone just keeps switching names. The short answer is yes, they're the same thing. The longer answer explains why two names exist at all.
RBL stands for Realtime Blackhole List. It was the original term, coined by Paul Vixie when he built the first blocklist of its kind. The "blackhole" part referred to routing traffic from listed IPs into oblivion.
DNSBL stands for DNS-based Block List. It describes how these systems actually work: your mail server performs a DNS lookup against the blocklist's domain. If the IP you're checking is listed, you get a response back. If it's not, you get nothing. The whole thing runs over DNS, which is fast and doesn't require a direct connection to the blocklist operator.
Over time, two things shifted the language. First, "blackhole" fell out of favor as a technical description since modern servers reject or filter listed mail rather than silently dropping it. Second, "blacklist" gave way to "blocklist" as the preferred term. DNSBL stuck around as the technically accurate label for the lookup mechanism.
In practice, you'll still see RBL used interchangeably with DNSBL in old documentation, firewall configs, and forum posts. Neither is wrong. They describe the same DNS-query-based system, just from different eras of email history.
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