Lookup for "551 5.2.4"
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 (5.2.4)
Mailing list expansion problem
The mailbox is a mailing list address and the mailing list was unable to be expanded. This code may represent a permanent failure or a persistent transient failure.