how can I make postfix more secure?
 

right now it seems like anyone can telnet into my server and send out. Is there a way so it will require authentication? or maybe only a certain range of ip can connect?

Yes. You should only allow your internal network to connect. By default, it is configured that way. To test out, try to connect from the internet and try to send an e-mail to a domain that is not hosted on this machine. You should get a relay_denied error. If it relays, you must do something to prevent relaying. See /etc/postfix/main.cf