The spool priority doesn't seem to work as advertised. With these settings in the domain's Priority tab, I never see a message with a 4 priority regardless of the number of attempts.
The priority goes from 5 to 3 after the 2nd attempt. Let me know if I'm missing something.
FWIW, not sure why this is a domain setting since all the domains mix together in the spool. Maybe it should be a global setting (remove it from each domain) which would simplify things.