I posted a how to the last time I talked about Postfix here. I neglected to add that if you are using a firewall, which I highly recommend, then you should add an entry to your firewall rules regarding Postfix. I happen to use ufw as frontend for iptables. So just type:
#ufw allow Postfix
Check your ufw status by typing:
#ufw status verbose
or #ufw status numbered
Happy New Year! Cheers!