Originally published on the Merlonix blog . A DNS blocklist (DNSBL, sometimes RBL) is a published list of IP addresses a provider has flagged as a source of spam. Mail servers consult these lists in real time: if the IP that just connected is listed, the message is usually rejected outright or filed to spam. A single listing on a widely-consulted list — Spamhaus, SpamCop, Barracuda, UCEPROTECT — can quietly kill a large share of your mail, including the transactional kind you can least affo...