Lookup for "501 5.4.6"
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 (5.4.6)
Routing loop detected
A routing loop caused the message to be forwarded too many times, either because of incorrect routing tables or a user-forwarding loop. This is useful only as a persistent transient error.