DNS Black Lists (DNS-BL) can be used to help prevent spam email from reaching your users. This security feature allows you to specify several DNS blacklisting services (which maintain lists of servers known to relay spam) that will be checked each time someone tries to send a message to your server. If the connecting IP has been blacklisted by any one of these services, the message(s) will be refused or flagged according to the settings on the Options screen.
DNS Black Lists includes a White List for designating IP addresses that you wish to make exempt from DNS-BL queries. Before activating DNS-BL, you should make sure that your local IP address range is on the White List to prevent lookups on those addresses. "127.0.0.1" is exempt and therefore doesn't need to be added to the list.