Lookup for "551 4.4.2"
Temporary Error - This status code indicates a transient issue and may resolve itself.
Basic Status Code Details (551)
User not local; please try
Enhanced Status Code Details (4.4.2)
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.