Real-time Blackhole is a database of IP addresses known to send spam, accessible over DNS, with specific responses to determine whether a host should be rejected or allowed.
A Domain Name System-based Blackhole List (DNSBL) or Real-time Blackhole List (RBL) is an effort to stop email spamming. It is a "blacklist" of locations on the Internet reputed to send email spam. The locations consist of IP addresses which are most often used to publish the addresses of computers or networks linked to spamming; most mail server software can be configured to reject or flag messages which have been sent from a site listed on one or more such lists. The term "Blackhole List" is sometimes interchanged with the term "blacklist" and "blocklist". https://en.wikipedia.org/wiki/DNSBL
