This is not an actual SPAM DNSBL. This list contains IP's of open DNS resolvers. An open DNS resolver is a DNS that provides name resolution outside of its administrative domain.
According to the RBL website they check IP addresses by querying the IP's at most every three days.
Is the information on our site wrong or incomplete? You don't want us to monitor your blacklist? Or do you operate a similar blacklist? We would like to hear from you!