DNSRBL uses intelligent filters that scan a large number of mail feeds to determine if a message is SPAM, JUNK and whether the SMTP transaction conforms to standards.
DNSRBL is not a traditional spam trap. Junk (promotional) mail, viruses, phishing and other malicious mailings will cause a listing. In addition, DNSRBL is checking that the PTR record and HELO conform to standards. Authentication schemes are also checked (SPF/DKIM).