> First, I don't want mail from non-monks.
While I understand this concern, I don't really agree with it. As AgentM mentioned in
the root of this thread, being able to list brainpan@perlmonks.org on my resume would be
desirable. Making this be an esoteric form of perlmonk-only communication would render
this benefit null and void. However, I do agree that there should be some defining
characteristic from mail coming from non-permonks. I've never really done much with
sendmail, but if it would be possible to add NonPerlmonk=True to the message header
(and not including this field for perlmonks, lest headers be forged) this would give your
mail client's mail filters something to search for and deal with as you see fit.

> Fourth, I'm not wholly comforted by the thought of people showing up here simply so they can get
@perlmonks.org accounts... as seems inevitable.

You seem to be neglecting the fact that this would be true of your 'private message page'
method as well. In either case, making a perlmonk(ey) rise to a certain status before
being granted an email address would seem to solve the problem.