Delayed Message: Network is busy
The email message has not yet been delivered because the recipient's email provider did not respond to the sender's email provider's request to connect.
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.
4.4.1: 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 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.