How to configure postifx smtp relay in CentOS/RHEL 5 and 6

The post is to illustrate the steps to configure the smtp relay using postfix on CentOS/RHEL 5 and 6.

For relaying mail through another smtp server, /etc/postfix/main.cf file has to be configured with “relayhost” parameter which specifies the destination smtp server.

1. Install the postfix rpm if not installed already.

# rpm -qa|grep postfix

2. Take a backup of the default configuration:

# cp /etc/postfix/main.cf /etc/postfix/main.cf.bkp_default

3. Edit the /etc/postfix/main.cf and append/modify the following parameter on the configuration.

myhostname = [hostname-of-server]
relayhost = [IP Address/Hostname of relay server] 

4. Restart postfix service to make the changes effect.

# service postfix restart

5. Verify the mail relay using telnet/mail(mailx) commands.