Delayed Message: Mailbox has a mailing list system issue
The email message has not yet been delivered because the recipient's email address, which is a mailing list, had trouble forwarding the email message the list members.
Check that the recipient's email address is the correct address for the mailing list you wish to reach. If an error is found, correct it in contacts list or address book for next time.
Contact the administrator of the mailing list for help.
4.2.4: Official Definition
A persistent transient failure is one in which the message as sent is valid, but persistence of some temporary condition has caused abandonment or delay of attempts to send the message. If this code accompanies a delivery failure report, sending in the future may be successful.
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.