Real-time blackhole list (RBL) is a list used by mail servers to identify email spammers. A RBL is an accumulation of IP addresses whose owners refuse to stop the dissemination of spam. Mail servers use this information to reduce spam in one of two ways:

  1. Marking the message as spam.
    These are the emails you find in your junk/spam folder.
  2. Not delivering the message.
    These messages never make it to your email account.

The method chosen depends on which list(s) the IP address is associated with. Mail servers typically use a number RBLs.

RBLs are also know by the following names and are used interchangeably:

  • Realtime Blacklist
  • DNSBL – Domain Name System-based Blackhole List