To enable Postfix system on Mac OS X (Tiger):
# launchctl load -w /System/Library/LaunchDaemons/org.postfix.master.plist
To disable the Postfix system on Mac OS X (Tiger):
# launchctl unload -w /System/Library/LaunchDaemons/org.postfix.master.plist
This is part of the launchd startup scheme in Tiger.