[A6] list now auto-rejecting mail from non-subscribers

[Will Yardley]

Minor policy change.... announcing just in case it makes a difference to
anyone. Messages from non-subscribers (including if you accidentally
send from the wrong address) will now be rejected. You will need to
resubmit your message if you accidentally send a message to the list
from the wrong address.

This is at David's reqeust, because we're getting bombed with so much
spam these days (making his moderation duties quite a hassle).

w