Lookup for "535 2.4.6"
This status code indicates a successful delivery or other operation. Any details are simply notes.
Basic Status Code Details (535)
Authentication failed due to invalid or insufficient authentication credentials
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.