Hi all,  I don't know what list manager you use for this list.
It doesn't appear to be Majordomo.

I have run Majordomo and others. These are members only services and
if the email address doesn't match one in the member list, it is simply
ignored or returned as chosen by the moderator.

Many groups use a 2- or 3-letter preface for the subject.  These can be
filtered by the receiver so he/she can ignore posts of no interest.
"OT" (off topic) is a big one.

I have, for several years, maintained several groups on Yahoo's free
group service.  I get _no_ spam to the lists.  Occasionally, I get spam
attempting to join a group.  These group forums are members only and
can be moderated, if preferred (that is, a moderator can approve each
email).

I highly recommend the Yahoo service.  You do have to put up with a
short advert for Yahoo at the end of each post.  But, for the service
provided, it is worth it.  (If the user chooses digested, then they get
one email daily, with all posts and only on advert.)  For the most
part, after setting up the group, it's on auto-pilot.

Best Regards,
Mac