DNSBL
A DNSBL (DNS-based Blackhole List, also called an RBL) is a blocklist of IP addresses -- typically ones observed sending spam, running open relays, or otherwise associated with abuse -- published in a format mail servers can query using ordinary DNS lookups for speed. A mail server checks the sending IP against one or more DNSBLs in real time before accepting a message; a listing on a widely trusted DNSBL like Spamhaus can cause legitimate mail from that IP to be rejected or sent to spam, even for senders who did nothing wrong, which is especially common behind shared or CGNAT addresses.
By AdminKit team, Netbase JSC · Updated