What is RBL?

 
RBL is an abbreviation for Real-time Blackhole List. It is also known as DNSBLs (DNS-based block lists or “blackhole” lists).
It is a medium to prevent email spamming or email abuse. Based on DNS protocol, RBL are lists of IP addresses which are intentionally or are hijacked for spam activity.
Most of the email servers use the RBL of some well known international anti-spam organizations, such as spamhaus, Sorbs, and spamcop, etc.
They query the database of RBLs and block the emails which the IP is listed.
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