Message Not Delivered: Network issue with the receiving system
The message could not be delivered because a network problem prevented the recipient's email provider from delivering the message.
the sender's email program 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.
5.4.3: Official Definition
A permanent failure is one which is not likely to be resolved by resending the message in the current form. Some change to the message or the destination must be made for successful delivery.
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.