Lookup for "551 2.4.6"
This status code indicates a successful delivery or other operation. Any details are simply notes.
Basic Status Code Details (551)
User not local; please try
Enhanced Status Code Details (2.4.6)
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.