What's the correctly way to make Postfix run permanently on Lion (not server)
The following changes worked for me:


Remove the following two elements from /System/Library/LaunchDaemons/org.postfix.master.plist:


&lt;string&gt;-e&lt;/string&gt;
&lt;string&gt;60&lt;/string&gt;



This will stop it exiting after 60 seconds.
Add the following element as a child of the &lt;dict&gt;...&lt;/dict&gt; element in the same plist file:


&lt;key&gt;KeepAlive&lt;/key&gt;
&lt;true/&gt;


Reload the file:


sudo launchctl unload /System/Library/LaunchDaemons/org.postfix.master.plist 
sudo launchctl load /System/Library/LaunchDaemons/org.postfix.master.plist




postfix should now be running as a daemon.

The above works on Mountain Lion, Mavericks, Yosemite and El Capitan. For systems that have System Integrity Protection (El Capitan and later) there are two options:


Copying /System/Library/LaunchDaemons/org.postfix.master.plist to /Library/LaunchDaemons/org.postfix.custom.plist, changing org.postfix.master to org.postfix.custom and then unloading and loading as above.
Disabling SIP (using csrutil), making the edits and then re-enabling SIP.


Note: With High Sierra and Mojave, the configuration file is com.apple.postfix.master.plist instead of org.postfix.custom.plist.