Lookup for "251 2.7.1"
This status code indicates a successful delivery or other operation. Any details are simply notes.
Basic Status Code Details (251)
User not local; will forward to
Enhanced Status Code Details (2.7.1)
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.