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:

vi /etc/postfix/main.cf

 Look for: inet_interfaces

To bind postfix to all interaces, use:

inet_interfaces = all

 
For a single or multiple addresses do this:

inet_interfaces = 77.74.55.1,127.0.0.1

 
After changing config, restart postfix and use netstat -lpn to see if the change was successful.