A Uniform Resource Locator (URL), more correctly called a Uniform Resource Identifier (URI), is typically a web link in a spam email. A Spam URI Real-time Block List (SURBL) is a database of URIs that have appeared in spam emails.
SpamAssassin version 2.63 supports SURBLs via a plug-in, and from version 3.0 onwards, SpamAssassin supports SURBLs without the need for a plug-in.
These tests use a small amount of network I/O while querying the database. They are suited to parallel processing.