Log into your server as root, or any other user with the ability to sudo.
Find the line labeled: "smtp inet n......"
587 can be changed to match the port that you wish to use.
Save the file, and exit the text editor.
Run "/etc/init.d/postfix restart" to restart Postfix.
Add the new port to iptables by running: "iptables -A INPUT -p tcp --dport 587 -j ACCEPT", changing 2525 to the port you choose.
Run "/etc/init.d/iptables save" to save the rule.
Run "/etc/init.d/iptables restart" to apply the new rule.