5.4.1: 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.1
[RFC3463] (Standards Track) from the The Internet Engineering Task Force (IETF) definition for codes that end with X.X.1:
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.