What’s the difference between a DNSBL, RBL, and URI blocklist?

You've probably heard these terms thrown around when someone's emails stop delivering. They sound similar, and they're all called "blocklists," but they catch very different things. Getting listed on one doesn't mean you're listed on the other, and fixing one won't fix the other.

Here's what each type actually does.

DNSBL and RBL (IP blocklists)

DNSBL stands for DNS-based Blocklist. RBL stands for Realtime Blocklist. The names are often used interchangeably, and they both do the same thing: list IP addresses that have been flagged for sending spam. When a receiving mail server gets an incoming connection, it checks the connecting IP against these lists in real time. If your IP is listed, the connection can be refused before your email even gets a chance to be read. Examples include Spamhaus SBL and XBL, and Spamcop.

URI blocklist (domain blocklists)

A URI blocklist works differently. Instead of checking the sending IP, it scans the content of the email itself, specifically the domains that appear in links, From addresses, and other content fields. If a domain inside your email is flagged, filters can block or divert the message even if your sending IP is perfectly clean. This matters because you could be using a reputable ESP with a great IP reputation and still get filtered if a domain you're linking to has been flagged. Examples include the Spamhaus DBL, SURBL, and URIBL.

Why this distinction actually matters

Let's say your emails suddenly stop reaching inboxes. If you only check your IP and it's clean, you might think you're fine. But if a domain in your footer or a link in your campaign has been flagged on a URI blocklist, that's the real problem. You're chasing the wrong fix. The same works in reverse: a clean domain doesn't help if your IP is blocked at connection time.

And you can (and should) be checking both. Our free Blocklist Checker runs against the major lists for your domain or IP. Worth bookmarking if you send regularly.

If you're already listed and working through the removal process, the severity and reach of each blocklist varies a lot. Not all listings are equally damaging, which is worth knowing before you decide how urgently to act.