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.
Please read the original message again. Roger was asking if most MTA's would requeue messages for multiple recipients, so that he could serialize incoming mail (i.e. only the first recipient would be accepted in any given transaction). And at least my impression was that they _should_ by the RFC's but that isn't necessarily a good way to write your code.
One thing I forgot to point out was that if a MTA does requeue 4xx recipients, it may use its internal retry intervals as if the *entire* message had failed. Consequently, recipient #10 might not get delivered for some hours or even days later...
John
