I have spent days trying to figure out why Snow Leopard won't send mails with PHP mail(). After viewing Console log I found out that Postfix is sending messages but it's bounced on mail servers as blacklisted!
To be short, you can't send it locally. You have to use other mail server to relay messages. Best option is to use Gmail. Configuring Postfix to use Gmail was horror. After reading numerous articles about it finally solved problem and I wanted to share it. You have to do it all in Terminal, so fire up Utilities->Terminal.