What are DNS block lists (DNSBLs)?
DNS blocklists are a published set of IP addresses that are in a format that can be easily queried by computer programs. Generally, this is a list of IP addresses that should be avoided. For email purposes, the DNSBLs are a list of IP addresses that are known to be senders of spam and malware. For more information about DNSBLs, see the Wikipedia entry for DNSBLs.
