Real-Time Blackhole Lists (RBL) are a simple but effective way for organizations around the world, to share the location (in this case, the IP addresses) of email systems that are reputed to send email SPAM.

URIBL’s are exactly the same idea, the only difference is that rather than listing IP addresses, they list domain name (i.e. URI’s) of know or suspected SPAM or Phishing sites.

The most common implementation of these lists are distributed via DNS, and referred to as DNS-based Blackhole Lists (DNSBL). Distributing this data via DNS makes perfect sense; it’s a technology that already exists, and servers all already have access to. It’s fast, and the data (IP addresses and domains) is well suited for DNS.