Lookup for "501 4.7.1"
Temporary Error - This status code indicates a transient issue and may resolve itself.
Basic Status Code Details (501)
Syntax error in parameters or arguments
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.