DNSBL or DNS BLacklists is software mechanism that prevents spamming IP addresses to spam further. How a DNSBL works? Receiving mail server send simple DNS query with sender ip address to DNSBL and if it will receive response with record (mostly directed to 127.0.0.*) it will refuse comunicate with sender. On this page you can test if your IP address or mailserver is not listed on some of DNSBL.