http://www.boutell.com/boutell/usenet2.html
for some software that accomplishes this.
I haven't really looked at this, but I presume the basic idea is to
maintain a simple database of all contributors to the list (by email
address). Messages from first-time contributors don't get forwarded to
the list as a whole. Instead, a robot bounces such messages back to the
sender, along with a =very= friendly cover letter pointing to a FAQ or
an archive of past traffic, or something like that. Such senders are
politely urged to re-send their message if they don't think it's
addressed by these resources. Since this re-send is their =second=
message to the list, and the first caused their address to be added to
the database, the re-send gets silently forwarded to the list as a
whole.
Spam will presumably come from "new" contributors, who (hopefully) won't
bother to re-send, so we'll never see such mail. As a bonus, messages
of the form "Please add me to the mailing list" will presumably bounce
as well, thereby saving us from seeing that variety of annoyance. The
cover letter can include instructions for automatically subscribing to
cover this case.
In the case of mailing lists, it make sense to seed the database with
the list of current subscribers.
- John Burger
The MITRE Corporation