Lookup for "501 5.4.7"
Permanent Error - This status code indicates a permanent issue and is unlikely to resolve itself. You should contact the Email Service Provider.
Basic Status Code Details (501)
Syntax error in parameters or arguments
Enhanced Status Code Details (5.4.7)
Delivery time expired
The message was considered too old by the rejecting system, either because it remained on that host too long or because the time-to-live value specified by the sender of the message was exceeded. If possible, the code for the actual problem found when delivery was attempted should be returned rather than this code.