... configuration takes advantage of the Spamhaus Block List (SBL) and the Exploits Block List (XBL) querying sbl-xbl.spamhaus.org but you can use a different DNSBL, for example local rbldnsd instance of sbl-xbl (increasing query performance).
Spamhaus's DNSBLs are offered as a free public service for low-volume non-commercial use. To check if you qualify for free use, please see: Spamhaus DNSBL usage criteria (http://www.spamhaus.org/organization/dnsblusage.html)