Definition
A DNSBL (DNS-Based Blackhole List) is a list of IP addresses known to send spam, published as DNS records. Mail servers query these lists in real time for every inbound connection. The most important DNSBLs are Spamhaus ZEN, Barracuda, and SpamCop. A single listing on Spamhaus will cause Gmail and Outlook to reject mail from that IP.
Use the tool: IP Blacklist Checker — check dnsbl for any domain.