ORBITrbl is a DNSBL (DNS Based Black List - one of the many tools used by server administrators in the ongoing war on spam.) that used by ORBITnet Clients & it Servers, but can be used by any one for FREE.*

A DNSBL is a means by which an Internet site may publish a list of IP addresses that some people may want to avoid and in a format which can be easily queried by computer programs on the Internet. The technology is built on top of the Internet DNS or Domain Name System. DNSBLs are chiefly used to publish lists of addresses linked to spamming. Most mail transport agent (mail server) software can be configured to reject or flag messages which have been sent from a site listed on one or more such lists. from Wikipedia