At 14:41 +0200 on 10/29/2011, Peter J. Holzer wrote about Re:
If a 4xx wait: was received as a reply to a RCPT TO command, try to
resend this message to this specific recipient after the specified time.
OTOH: If this is a multi-recipient message, keep trying until you run
out of recipients (remembering which ones were rejected). This way if
one or wore were accepted these recipients will immediately receive
the message and it will then be requeued for a later attempt with a
trimmed recipient list.