You need to make sure that you have the port open on the firewall on the box that has apache server, postfix and dns( i think). but I have never had the router onto the internet port open for postfix or dns. There is one draw back if you send the email to email@example.com I will turn up in spam. and some isp will block it altogether. Because they can't talk to postfix. but if you are only using it on a form on one or two web sites and you choose the right email address it is fine. I would go gmail
I think from memory you also need dns but it is a while since I did it? if you have a look at some of you old post you will see i asked something similar. Don't laugh to much I was very raw.