One thing I've seen people due to work around this is have the list send out a
request the subscriber to fill in a bio or survey request to be submitted to
the list as an introduction with instructions as to how to return the bio to
the request-<listname> address in order to get their subscriptions approved.

The bio provides an aid in prescreening, plus you get a message from the
requester that includes their full headers.

Good luck,
Randy