A very good page on setting up Postix as a mail server on OS X, including fixing a broken launchd script.
Set the org.postfix.master.plist file exactly as it is on that page. If you leave out the last two keys, it might tell you that it was launched but you won’t be able to reach it.
Also, the following commands might tell you that the script has already been loaded:
cd /System/Library/LaunchDaemons
sudo launchctl load org.postfix.master.plist
In that case, simply do
cd /System/Library/LaunchDaemons
sudo launchctl unload org.postfix.master.plist
sudo launchctl load org.postfix.master.plist