I also struggled to find a way to advise folks with each message how
to respond to the <from:> and post mail back to the list.  The answer was
in that bottom_banner mail template.
 
For a list titled <[log in to unmask]>
the command back to LISTSERV was:
 
PUT CHAPPER MAILTPL PW=xxxxxx
>>> BOTTOM_BANNER
.FO OFF
Please send all mail to <[log in to unmask]>
.FO ON