Apple ships Mac OS X by default with all necessary software to make postfix run with SASL (Simple Authentication and Security Layer) and TLS (Transport Layer Security). In the following link I have written a short guide how to enable SASL and TLS in postfix:
Postfix + TLS + SASL on Mac OS X Panther 10.3.3Mac OS X Hints
http://hints.macworld.com/article.php?story=20040426145743653