SURBL stands for Spam URI Realtime Blocklists.
Most realtime blocklists try and block the email addresses that spammers send from. As spammers can generate (or fake) a unique email address with each message, this isn't a very good defence. SURBL blocks the URIs they point to inside the body of the email, and as such can have a much higher success rate.
Open Relay Filter.One page links to SURBL:
