The order in which the H lines appear determines their order in the final message. M Contains a message printed by the mailq command. It is mainly used to store status information. N Specifies the total number of delivery attempts. Used for Delivery Status Notifications only.
P Contains the priority of the current message.
The priority is used to order the queue. Higher numbers mean lower priorities. The priority increases as the message sits in the queue. The initial priority depends on the message class and the size of the message. Used exclusively for Delivery Status Notifications.