Unfortunately, LISTSERV TIPS is very out of date on some things...and this
is one of them.  :)  The file you want is called DEFAULT MAILTPL, and the
beauty of the change is that LISTSERV now allows a listowner to manipulate
the mail templates without intervention from the LISTSERV maintainer.
 
The change was made in LISTSERV version 1.8a and is documented in the
1.8a release notes.
 
Nathan