This is based on a subscriber settable option, with
a list keyword default.  The most efficient for
LISTSERV, its host machine, and "the network" is
to use SHORT, which typically is seen by the
user as "MULTIPLE RECIPIENTS OF LIST listname"
 
Take a look at the LISTSERV Reference Card.  Don't
have one, issue the command
 
INFO REFCARD
 
/Pete Weiss at Penn State