Lookup for "220 5.1.6"
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 (220)
Enhanced Status Code Details (5.1.6)
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.