Message Not Delivered: Network is busy
The message could not be delivered because the recipient's email provider did not respond to the sender's email provider request to connect.
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.
5.4.1: 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 outbound connection attempt was not answered, because either the remote system was busy, or was unable to take a call. This is useful only as a persistent transient error.