A DNSBL is a published list of the IP addresses of known spam sources that is used as one factor in deciding to identify an email as spam.
A DNS-based blackhole list is a means of publishing a list of IP addresses, in a format that can be queried via DNS. DNSBLs are chiefly used to publish lists of addresses linked to spamming. Most MTA software can be configured to reject or flag messages that have been sent from a site listed on one or more such lists.