Would the sending server DATA for the first recipient and reconnect for the next ones?
If memory serves, there are MTA's in the wild (old Lotus Notes I think it was) which would freak out something fierce in response to 4xx in the RCPT TO: phase. On the other hand, some spammers refuse to take the hint, so I've been thinking of altering dnsbl to return DENY_DISCONNECT instead of just DENY (or rather to add a parameter to switch that behavior on at will).
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. But, accepting for all is more conservative. You could add a custom plugin which notes which recipients would have denied the message and then remove that RCPT in data_post (or is it post_data?).
HTH
John
