DNS black lists are databases of known spammers. These databases contain IP addresses that are known to send spam. They also contain IP addresses that have open mail relays, because a spammer can easily use these systems to send out spam.
IMail Server uses DNS black lists during connection filtering. In order to fully understand how antispam and connection filtering work, it is necessary to understand DNS black lists. Connection filtering compares each message against the configured DNS black lists to see if the IP address of the connecting server is listed. If the result is positive, the message is either deleted or an X-Header is inserted into the message.
You can separate DNS black lists into two categories: standard DNS Black Lists and trusted DNS Black Lists.
A trusted DNS black list is one that you know is updated frequently, and is more likely to be accurate. You may also identify a black list as trusted because you find that for your uses it produces the least number of false positives.
Warning: If a message makes a match on the Trusted Black List, it is automatically deleted.
A standard DNS black list is a black list of which you are uncertain about its accuracy. If a message matches one of these lists, an X-Header is inserted into the message, indicating which black list it matched.
DNS black lists are configurable for the entire server, which enables a system administrator to decide which DNS black lists are available to each domain. Each domain administrator is then responsible for enabling the configured black list for the domain. A domain cannot use a black list that is not configured and enabled for the server.