kawakami: > And this problem occurs NOT always, only in following situatision, > > > 2, A message was sent on IPv4, but resuted 451 error. > > 3, A message was sent on IPv6, but resuted 451 error.
After a 4xx error, the SAME Postfix SMTP client process may
IMMEDIATELY try to make the SAME delivery to a different MX host.
If this is new, please read the SMTP protocol spec, RFC 5321.
Wietse
