Lookup for "500 2.1.6"
This status code indicates a successful delivery or other operation. Any details are simply notes.
Basic Status Code Details (500)
Syntax error, command unrecognized [This may include errors such as command line too long]
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.