By default, Postfix will listen on all IPs on a server. In the case of multi-homed servers, you may want to force Postfix to use a particular IP because of reverse dns, whitelisting, etc.
To do this, add the following to main.cf:
smtp_bind_address = x.x.x.x
You can the full details at http://www.postfix.org/postconf.5.html#smtp_bind_address.