Hrm, makes me think that one way of doing it might be to use a hashed value of ( the email address plus a secret key ) in order to specify the recipient. Those who want to receive mail at a specific address via the form could be given the hashed value. Although it obscures the actual destination, it's no protection against a spammer who doesn't care who's on the other end (which is, of course, the overwhelming majority of spammers).
If not P, what? Q maybe?