Message Not Delivered: Unknown issue with network connection
The message could not be delivered because of an unknown network connection problem between the email providers.
Fixes
The email provider is working to resolve the network issue. Typically, these issues require the providers to fix them.
Consider contacting the recipient through an alternative email address or other means.
5.4.0: Official Definition
[RFC3463] (Standards Track) from the The Internet Engineering Task Force (IETF) definition for codes that start with 5.X.X:
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.
[RFC3463] (Standards Track) from the The Internet Engineering Task Force (IETF) definition for codes that end with X.4.0:
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.