Lookup for "251 4.7.1"
Temporary Error - This status code indicates a transient issue and may resolve itself.
Basic Status Code Details (251)
User not local; will forward to
Enhanced Status Code Details (4.7.1)
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.