I don't know if any mailing list software support this:

- list unmoderated for subscribers

- if a non subscriber sends an email, auto reply
to the sender with message in copy plus a note
giving a one shot time-limited random key to put in the subject
or body and an invitation to resend.

- if a non subscriber sends an email with
a valid key, the key is cancelled and the message
is posted to the list. They key could stay
valid for one week to allow some discussion
without too much hassle for the sender.

Otherwise I'm not bothered enough by spam, no strong opinion.

Laurent Guerby <[log in to unmask]>