An RBL (or DNSBL) is a DNS-queryable list of spamming IPs — Spamhaus being the most influential. Receiving servers consult them on every connection; listed IPs get refused or spam-foldered en masse.
Shared servers land on RBLs through one compromised mailbox or script — and every account on the IP suffers together. Routine listing checks turn “customers say mail bounces” into an early, specific alert; the cure is stopping the spam source first, then requesting delisting.
Frequently asked questions
How do I know if my server IP is blacklisted?
Query the major lists (or let a scheduled check do it). Bounce messages also usually name the blocking list outright.
How long does delisting take?
From hours to days depending on the list — but only after the spam source is stopped, or the listing returns immediately.