Warum gibt es noch immer Mailserver-Admins, welche die RFC 3463 - Enhanced Mail System Status Codes nicht kennen (http://www.ietf.org/rfc/rfc3463.txt)? :'(
5.XXX.XXX Permanent Failure
A permanent failure is one which is not likely to be resolved
by resending the message in the current form. Some change to
the message or the destination must be made for successful
delivery.
X.7.XXX Security or Policy Status
The security or policy status codes report failures involving
policies such as per-recipient or per-host filtering and
cryptographic operations. Security and policy status issues
are assumed to be under the control of either or both the
sender and recipient. Both the sender and recipient must
permit the exchange of messages and arrange the exchange of
necessary keys and certificates for cryptographic operations.
X.7.1 Delivery not authorized, message refused
The sender is not authorized to send to the destination. This
can be the result of per-host or per-recipient filtering. This
memo does not discuss the merits of any such filtering, but
provides a mechanism to report such. This is useful only as a
permanent error.