DNSBL/RBL uses the DNS protocol to query the well-known IP addresses which have been complaint to send worm, virus & junk mail. Using RBLs to prevent junk, virus & worm email is very effective. The algorithm of RBLs is different with that of Anti-Spam and Anti-Virus services as RBLs works based on IP address, while Anti-Spam and Anti-Virus services work based on the message content.

The nsZones (RBL) utilizes a standard DNSBL (Domain Name System Block List) implementation that can be used to reduce spam volume on many popular email systems. DNSBL systems list IP addresses, often those that have been observed by the list operator to be sending spam or hosting spammers. By adding DNSBL entries to an email server configuration, those email servers can either reject connection attempts from listed IP addresses or use that listing to apply appropriate filtering policy.

It is important to note that a DNSBL cannot stop anyone from sending email. A DNSBL only prevents delivery at the receiving end at the receiver's instruction. DNSBLs are strictly defensive tools, and they cannot do any offensive damage such as denial of service attacks.

SURBL is an abbreviation for Uniform Resource Identifier Blacklist. A SURBL lists domain names that appear in URIs such as web sites mentioned in message bodies. This is similar to a DNSBL but it lists domain names rather than IP addresses.

The DNSBL / RBL can be queried directly as bl.nszones.com