5.4.0: Official Definition
5.X.X
[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.
X.X.0
[RFC3463] (Standards Track) from the The Internet Engineering Task Force (IETF) definition for codes that end with X.X.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.