Would the sending server DATA for the first recipient and reconnect for the next ones?
The RFC's definitely require that a 4xx in the RCPT TO: phase be treated as a temporary error for that recipient only (think disk quotas), so any MTA that doesn't requeue is broken.
Consider also that some recipients may not exist (account deleted, typo in original sender's message).
Why not just send a 5xx for the specific recipient? I don't know why you would want a 4xx if you're going force a reconnect just to reject it anyway.
I eventually plan to get to a per-user config (like Gavin built) that would allow some users to get everything (spam trap) and others to block all of it, and this would certainly help that.
-- Bryan
