Delayed Message: Unknown issue with network connection
The email message has not yet been delivered because of an unknown network connection problem between the email providers.
Contact the sender's email or network support team as there was a network issue, possibly with the sender's email server. This is typically something that needs helpdesk support to resolve.
Also, consider contacting the recipient through an alternative email address or other means.
4.4.0: 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.
Something went wrong with the networking, but it is not clear what the problem is, or the problem cannot be well expressed with any of the other provided detail codes.