I know that it would be unfortunate, but I think it is time to try and put a filter on all mail getting posted to this mailing list, and at least filter out anybody who has not signed up for the list.

This is unfortunate, because there can be good suggestions from people who are simply lurking or even reading the mailing list archive, or even more where you may have more than one e-mail account and you want to send a message (say from home and work) that has different mail servers. On the positive side, however, this tends to keep idiots off the list that simply find an e-mail address and send a pile of spam to that address. Just my 2 cents.