Message Not Delivered: Message priority issue
The message could not be 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.
5.3.6: Official Definition
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.
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.