I can start Postfix temporarily with:
$ sudo launchctl launchd% start org.postfix.master
However, Console.app shows that it only runs for a short time:
26/11/11 2:00:55.710 PM postfix/master: master exit time has arrived
I think the file /System/Library/LaunchDaemons/org.postfix.master.plist needs to be updated to make Postfix run permanently, but I haven't found the answer yet.
What's the correct way of getting Postfix starting at boot time and running permanently?
Update In order to edit the file as recommended by the selected answer, I first converted it to XML.
plutil -convert xml1 /System/Library/LaunchDaemons/org.postfix.master.plist
Then just edited it with Vim:
sudo vim /System/Library/LaunchDaemons/org.postfix.master.plist
I had issues trying to get Xcode to edit the file.