What is "surbl"

SURBL (previously stood for Spam URI RBL) is a collection of URI DNSBL lists of Uniform Resource Identifier (URI) hosts, typically web site domains, that appear in unsolicited messages. SURBL can be used to search incoming e-mail message bodies for spam payload links to help evaluate whether the messages are unsolicited. For example, if http://www.example.com is listed, then e-mail messages with a message body containing this URI may be classified as unsolicited. URI DNSBLs differ from prior DNSBLs, which commonly list mail sending IP addresses. SURBL is a specific instance of the general URI DNSBL list type.