Lookup for "551 5.4.5"
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 (5.4.5)
Mail system congestion
The mail system was unable to deliver the message because the mail system was congested. This is useful only as a persistent transient error.