4.3.6: 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.
The message was accepted for relay/delivery, but the requested priority (possibly the implied default) was not honoured. The human readable text after the status code contains the new priority, followed by SP (space) and explanatory human readable text.