4.4.3: 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 network system was unable to forward the message, because a directory server was unavailable. This is useful only as a persistent transient error. The inability to connect to an Internet DNS server is one example of the directory server failure error.