DNSBL
A DNSBL (DNS-based Blackhole List, Block List, or Blacklist; see below) is a list of IP addresses published through the Internet Domain Name Service in a particular format. DNSBLs are most often used to publish the addresses of computers or networks linked to spamming; most mail server software can be configured to reject or flag messages which have been sent from a site listed on one or more such lists.
"DNSBL" is a software mechanism, rather than a specific list or policy. There are dozens of DNSBLs in existence which use a wide array of standards for listing and delisting of addresses. These may include listing the addresses of zombie computers or other machines being used to send spam, listing the addresses of ISPs who willingly host spammers, or listing addresses which have sent spam to a honeypot system.
