I consider the following a bug.  Others may call it a "feature".

When someone posts with no subject at all, LISTSERV passes the
post through without inserting the '[listname]' into a Subject:
header.  This, of course, messes up mail filters based upon
expecting to see 'Subject: [listname]...' with a possible "Re:"
in between ":" and "[listname]".

I suggest that if no Subject: is present, LISTSERV should insert
something like:
        Subject: [listname] <no subject>
for SUBJECTHDR subscribers.  IETFHDR subscribers should get no
such thing, of course.  Other subscribers maybe ought to get
        Subject: <no subject>
but I'm not as adamant about that.

The case I'm talking about is when the header is actually absent,
not just blank.  However, it would be reasonable to do the same
thing if it's there but blank.  (The latter case causes some mailers
to generate "Re: your mail" which is confusing as heck on a mailing
list!  "Re: <no subject>" would be just fine.)

Cheers,
Stan