Jeremy C. Reed wrote:
> Maybe the list server should email a non-subscriber back and ask them to 
> reply back to allow their posting to be accepted.

I hope you don't mean sending it back with the message body because this would
make a nice spam relay then. Even if not, these would be send to people which
have not send the spam and these kind of mails are actually worse than spam.

You don't have to treat each list the same. The lists which are mostly used by
subscribers can be changed to read-only for non-subscribers and the others
could be handled as usually.

-- 
Christian