SURBLs are lists of Uniform Resource Identifier (URI) hosts, typically web site domains, that have been observed in unsolicited messages. SURBLs differ from most other RBLs in that they're used to detect spam based on message body URIs (usually web sites). Unlike most other RBLs, SURBLs are not meant to identify spam senders by their message headers or connection IP addresses. Instead they allow you to identify messages by the spam sites mentioned in their message bodies.