Lookup for "251 4.2.4"
Temporary Error - This status code indicates a transient issue and may resolve itself.
Basic Status Code Details (251)
User not local; will forward to
Enhanced Status Code Details (4.2.4)
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.