Actually it is no so easy to restart Postfix in Mac OS X 10.8 Mountain Lion like in Linux :-)
However you can restart Postfix by plist unload / reload:
sudo launchctl unload /System/Library/LaunchDaemons/org.postfix.master.plist
sudo launchctl load System/Library/LaunchDaemons/org.postfix.master.plist
By the way, if you have a problem with Postfix, look at this article: Cannot send mail in Mac OS X 10.8 Mountain Lion