4.4.6: 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.
A routing loop caused the message to be forwarded too many times, either because of incorrect routing tables or a user- forwarding loop. This is useful only as a persistent transient error.