How edit org.postfix.master.plist?

In order to use smtp (via Gmail) from within a localhost apache2 server, I need to edit /System/Library/LaunchDaemons/org.postfix.master.plist.

However, no matter what editor I use, starting it with "sudo", I cannot save the file after adding some lines to it.

For example, using "sudo vi", when I try to save the file with command :wq I get error:

E45: 'readonly' option is set (add ! to override

Even if I do try to save instead with command :wq!, I still get a similar error (E212: Can't open file for writing.

I note that I find:

[me:/etc/postfix]$ ls -l /System/Library/LaunchDaemons/org.postfix.master.plist

-rw-r--r-- 1 root wheel 572 Jul 31 2015 /System/Library/LaunchDaemons/org.postfix.master.plist