In this exercise we will setup sending mails but this will be filtered with DNS record. I hope that you don't remove your configuration for DNS which we made before. On both machine you should setup that themselves use your DNS. According to this you should change /etc/resolv.conf

search my domain
nameserver IP address for name server

to be sure this is working I will start next command:

[root@security2 ~]# host client has address

You should add rule to DNS, so in setenforce.internal (look on next link DNS setup) add next: IN A

and in file you should add next:

FEATURE(`dnsbl', `', `"550 Message from "$&{client_addr}" blocked." ')dnl

Restart both service and try to send mail from security2. Now you will got something like this:

550 5.7.1 Message from blocked.