What is SURBL(Spam URI Real time Block list)?
SURBL differs from most other RBLs in that its used to block messages based on the domain names in message body URIs (usually web sites), for example, those which have been previously reported to SpamCop. Some of the recent domains are visible on SpamCop's Spamvertised sites page. So SURBL is not used to block spam mail servers like most other RBLs; instead it allows you to block messages based on spam domains that occur in the message body.
For more information go to http://www.surbl.org