Delayed Message: Network issue with the receiving system
The email message has not yet been delivered because a network problem prevented the recipient's email provider from delivering the message.
The sender's email system will retry this message a few more times and provide further updates.
if the sender is not notified within a few days that the message was delivered, try sending it again. Or contact the recipient another way, such as phone or text message.
4.4.3: 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 network system was unable to forward the message, because a directory server was unavailable. This is useful only as a persistent transient error. The inability to connect to an Internet DNS server is one example of the directory server failure error.