exitzero.org is a blacklist containing IP addresses that have attacked honeypots or tried to send mail to an invalid address on honeypot mail servers. To use the blacklist craft a standard rbldns DNS query for your IP and query bl.exitzero.org you will receive 127.0.0.2 for an IP address that has tried to authenticate and 127.0.0.3 for an attempt to send mail to an invalid email address.