Run the following the check if postfix is listening on port 25/tcp:
netstat -plnt |grep :25
You should see a line like:
tcp 0 0 0.0.0.0:25 0.0.0.0:* LISTEN 2460/master
If you don't see such a line, check /etc/postfix/master.cf.
In that file, you should see a line like:
smtp inet n - n - - smtpd
The first column is the port, which postfix derives from /etc/services.
You should look up general documentation on setting up Postfix, so your server doesn't become a spam relay. The CentOS wiki has fairly good documentation in that regard.
Also, look at http://www.postfix.org/postconf.5.html#inet_interfaces which will describe how to configure Postfix to listen on different network interfaces. The relevant line will be found in /etc/postfix/main.cf.