found the following:
http://www.faqs.org/rfcs/rfc1893.html
5.X.X = Permanent Failure
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.
hope this helps