SURBLs are lists of web sites that have appeared in unsolicited messages. Unlike most lists, SURBLs are not lists of message senders.
Web sites seen in unsolicited messages tend to be more stable than the rapidly changing botnet IP addresses used to send the vast majority of them. Sender lists like zen.spamhaus.org can be used in a first stage filter to help identify 80% to 90% of unsolicited messages. SURBLs can help find about 75% of the otherwise difficult, remaining unsolicited messages in a second stage filter. Used together with sender lists, SURBLs have proven to be a highly-effective way to detect 95% of unsolicited messages.