Here's another issue with the Spool Priority -- it seems to be ignored for local deliveries.
- Set the Sender Priority Override for a mailbox to 3.
- Using that mailbox, send a newsletter to a few thousand users on your server which go into spool with a priority=3.
- Send yourself a message from @gmail which gets added to spool with a priority=5.
- It takes 10 minutes to deliver the newsletter, then you receive the @gmail message.
Expected behavior: the @gmail message with a priority of 5 would get delivered sooner.