Configure Postfix Server as a Relay on CentOS

Install Postfix and ensure that the service is enabled:

# yum install postfix cyrus-sasl-plain
# systemctl enable postfix

In case Sendmail is installed, remove it:

# yum remove -y sendmail*

Configure Postfix Server as a Relay

Note that this section applies to both servers. Open /etc/postfix/main.cf for editing and configure the following:

relayhost = mail1.goyun.info
fallback_relay = mail2.goyun.info

Create a new file /etc/postfix/sasl_passwd and add the authentication details of the external SMTP server:

mail1.goyun.info:587 goyun.info@my-repay-server.com:passwd

Prevent non-root access:

# chmod 0600 /etc/postfix/sasl_passwd

Create a database file:

# postmap /etc/postfix/sasl_passwd

Check configuration and restart Postfix service:

# postfix check
# systemctl restart postfix

Test it:

# echo test|mailx -s TEST receiver@goyun.info