To ensure the security of your system, you need to prevent the connection of users who might abuse your e-mail server.
Unwanted users might connect to your system, and send unsolicited mail. This unsolicited e-mail takes a great amount of processing unit cycles and space. Also, if your system allows others to relay unsolicited mail, other systems might block the mail that comes from your system.
You can specify IP addresses of known unwanted users, or you can connect to a host that contains a Realtime Blackhole List (RBL) server. These Realtime Blackhole Lists provide a listing of known IP addresses that send unsolicited mail.
To specify known IP addresses or a host with a Realtime Blackhole List...
Related concepts
Controlling e-mail access