Lookup for "551 5.4.2"
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 (551)
User not local; please try
Enhanced Status Code Details (5.4.2)
Bad connection
The outbound connection was established, but was unable to complete the message transaction, either because of time-out, or inadequate connection quality. This is useful only as a persistent transient error.