To configure postfix to relay mail using another MTA, you may do the following steps:
postconf -e 'relayhost = smtp.to.relay.com'
postconf -e 'smtp_sasl_auth_enable = yes'
postconf -e 'smtp_sasl_password_maps = hash:/etc/postfix/sasl_passwd'
postconf -e 'smtp_sasl_security_options='
Replace smtp.to.relay.com with the original MTA hostname that you going to use for relaying. Now, create the sasl_passwd file in /etc/postfix with the following inside:
smtp.to.relay.com smtp_username:smtp_password
Now, use postmap to generate postfix hash db:
postmap /etc/postfix/sasl_passwd
You can verify if it’s working with the following:
postmap -q smtp.to.relay.com /etc/postfix/sasl_passwd
This will return the username and password for your smtp relay host.
Now all you need to do is to restart the postfix
service postfix restart