"why does that matter? i have a hostname and a server box. isn't that all i need??? ftp, http, mysql, ssh, etc all work okay. why can't i get incoming internet mail as easy?"
OK, you are going to have to be more humble from now on, because you are so ignorant you don't even know how sad it looks. Shutup and listen.
For email servers you need to have your DNS record set with the proper MX record point to your DNS. Mail will not now or ever route to your mail server simply because you started postfix on your box.
The internet does not give one shit about your mail server. You need to put the entry into the DNS MX record for anyone to know that you ever existed. And dynamic DNS services do not do this by default. You must pay for this- with real money. I hope you are grasping the concept. Good luck.