If you need to relay without authentication (or just IP based authentication) you can edit Postfix configuration file in /etc/postfix/main.cf and add the following line:
Code:
relayhost = <SMARTHOST>:<PORT>
Then restart Postfix: /etc/init.d/postfix restart
If you need to relay with authentication you'll also have to create a password maps file in addition to the step above.
Create a password file nano /etc/postfix/sasl_passwd and add the following code.
Code:
<SMARTHOST> <USERNAME>:<PASSWORD>.
Run the command postmap hash:/etc/postfix/sasl_passwd and restart Postfix (/etc/init.d/postfix restart).