Also a pretty basic thing, here you are:
Edit the main.cf with your favorite editor, for me that is vi and on Debian the file path goes like that:
Look for: inet_interfaces
To bind postfix to all interaces, use:
inet_interfaces = all
For a single or multiple addresses do this:
inet_interfaces = 126.96.36.199,127.0.0.1
After changing config, restart postfix and use netstat -lpn to see if the change was successful.