|Either via a List Keyword (preferrable) or by using an exit routine?

With EXITS, all things are possible; but basically native mode does NOT
filter on arbitrary Subject:s.  There are some RESERVED subjects that
LISTSERV(R) won't distribute due to their being highly indicative of an
automated response back to the list.

Another special case is the TOPICS feature.  See my attachment.
|If not, is it being considered for the next version of Listserv?

Pete Weiss at Penn State