We are setting up Centos 7 running MailScanner with postfix as mail gateway accepting mail for domain aaa.com, bbb.com, ccc.com and relaying mail for any SMTP server located on 102.168.0.0/24 subnet or from 192.168.2.2. Firewall and system requirements CentOS 7 minimal install Inbound – tcp port 25 Outbound – tcp ports 2703, 7, udp port