The blacklist check will show if the entered IP address is listed with DNSBL systems. These systems are used by email system administrators to try and eliminate spam email messages before they reach their users. DNSBL is a Domain Name Blacklist. There are dozens of DNSBLs online, all of which use a wide variety of criteria for listing and delisting addresses. Spam filters utilize various DNSBLs to verify that the incoming emails do not originate from blacklisted sites. DNSBLs are generally the first line of defense against spam.