Delayed Message: Message priority issue
The email message has not yet been delivered as the priority level the sender's email program specified for this message was not honored.
The message was delivered, there is no action other than to understand that the priority requested could not be applied.
If you get this type of delivery notice repeatedly consider checking whether the sender's email program is set to assign a high priority to all of the messages. Otherwise, contact the email provider for help.
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.