Delayed Message: Overloaded network
The email message has not yet been delivered because the email system was unable to deliver the message due to recipient's email network being congested or overloaded.
Try waiting a bit since the mail server is busy. The sender's email provider will retry this message a few more times to see if the message gets through and provide further updates.
If this error continues, consider contacting the recipient through an alternative email address or other means.
4.4.5: 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 mail system was unable to deliver the message because the mail system was congested. This is useful only as a persistent transient error.