The DNS host hMailServer should query when doing a DNS blacklist lookup.
The IP address that the DNS blacklist server will return if the senders IP address is found in the DNS blacklist. It's possible to use wildcards in the IP address.
There is some limited support for specifying multiple possible addresses. You can separate multiple addresses by using |, such as 127.0.0.1|127.0.0.2. In the last section of the IP address, you can use - to specify a range, as in 127.0.0.2-8. So if you specify 127.0.0.2-8|127.0.0.10-11, that will cover the IP adresses 127.0.0.2, 127.0.0.3 up until 127.0.0.8 and 127.0.0.10 and 127.0.0.11.
The message that hMailServer will give to the sending client if an email is rejected.