You hit send, your client says "delivered," your recipient says "nothing arrived." Nine times out of ten it's in their spam folder. Eight times out of those nine, an IP-based blocklist is the reason.
What an RBL is
RBLs (DNS-based Realtime Blackhole Lists) are public lists of IPs that have sent spam, hosted malware, or behaved badly. Receiving mail servers query a handful of RBLs for every incoming connection; if the sending IP is on any of them, the message gets routed to spam or rejected outright.
Check your sending IP
Our IP Blacklist Check queries 25+ public RBLs in one shot. If you're listed on one or two minor lists, you're fine. If you're on Spamhaus, SpamCop, or Barracuda, you have a real problem.
Getting off a list
Each RBL has its own delisting process. Spamhaus accepts requests after you've identified and fixed the underlying problem (compromised account, misconfigured mail server, open relay). SORBS automates expiry. Some smaller lists never delist.
Prevention
Set up SPF, DKIM, and DMARC correctly. MX Lookup + DMARC Analyzer will tell you which signals are missing.