There are network subnets that have been taken over by Spammers and run by bots. These networks are recorded and documented by Spamhaus and provide you a quick way to modify your firewall to eliminate these know blocks of Spam. You will need to have an iptables firewall and add this section to the firewall which will use the information found in the list to drop the subnets thus taking the load off your Postfix mail server.