DNSBL / RBL (Blocklist)

A DNSBL (DNS-based Blocklist), also called an RBL, is a published list of IP addresses or domains flagged for sending spam or malicious traffic.

Mail servers and security systems query DNSBLs to decide whether to accept or reject traffic from a given IP. If your sending IP lands on a major blocklist, your emails get rejected or sent to spam, often without any bounce you would notice.

Blocklist monitoring checks your IPs and domains against well-known lists and alerts you the moment you are listed, so you can request delisting before deliverability collapses.