Lookup for "535 2.1.6"
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 (2.1.6)
Destination mailbox has moved, No forwarding address
The mailbox address provided was at one time valid, but mail is no longer being accepted for that address. This code is only useful for permanent failures.