Configuring Postfix as a Local Network Relay

This howto assumes that the relay server’s IP address is 192.168.1.22 and is running RHEL/CentOS 7. Only mail from the 192.168.1.0/24 network should be accepted and relayed.

Install Postfix

[root@rhce-server ~]# yum install postfix

Configure Systemd

[root@rhce-server ~]# systemctl enable postfix
[root@rhce-server ~]# ^enable^start

Configure the Firewall

[root@rhce-server ~]# firewall-cmd --add-service=smtp
success
[root@rhce-server ~]# firewall-cmd --add-service=smtp --permanent 
success

Configure Postfix

Postfix’s main configuration file is located at /etc/postfix/main.cf.
Configure Postfix to listen on the correct interface.

inet_interfaces = all

Configure the trusted network.

mynetworks = 192.168.1.0/24

Configure the list of domains that this Postfix service should consider itself the final destination for. In my case the server is named rhce-server.

mydestination = rhce-server, localhost.localdomain, localhost

Configure all mail not destined for this server to be relayed to another SMTP server. I am using Time Warner Cable’s SMTP server for Northeast Ohio. The brackets tell Postfix to turn off MX lookups.

relayhost = [smtp-server.neo.rr.com]

Restart Postfix

[root@rhce-server postfix]# systemctl restart postfix

Send a Test Email

[root@rhce-server postfix]# mail -s "rhce-server test" josh@example.com
testing our null postfix configuration
.
EOT

With any luck we should be all set. You can verify the mail was successfully relayed in /var/log/maillog.