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.
Using SURBLs requires a mail filter that can extract web sites from message bodies and check them against the lists. Many applications support SURBLs, including SpamAssassin and filters for most major MTAs including sendmail, postfix, qmail, exim, Exchange, qpsmtpd and others. For a partial list of dozens of applications supporting SURBLs, please see the Links page. Note that direct blocking at the MTA level is not recommended. It's generally better to use SURBLs along with multiple, weighted factors, as SpamAssassin does. For new implentations, please see the Implementation Guidelines.