Lookup for "551 2.2.4"
This status code indicates a successful delivery or other operation. Any details are simply notes.
Basic Status Code Details (551)
User not local; please try
Enhanced Status Code Details (2.2.4)
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.