What is a blocklist operator?

A blocklist operator maintains lists of IPs, domains, or URLs associated with spam or abuse. Providers check these lists to decide whether to accept or reject mail. These lists are formally called DNSBLs or RBLs. Major operators like Spamhaus, Invaluement, and SURBL filter billions of unwanted messages every day. They are the maritime authorities marking dangerous ships and preventing them from entering safe waters.