Strictly speaking 5.7.1 is
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.
In other words, it looks like you've been filtered, either at the host
level or at the user level.
Nathan