The Level 10 reward is a Pii dungeon much like http://puzzledragonx.com/en/mission.asp?m=1653
The unopened email is valid for a year, but the dungeon is only available temporarily once opened. It disappears once you complete it, but you can easily spend an hour leaving and reloading it until you get the Py you want.
It only costs 10 stamina to load, but if you have anything like my incorrigible luck it might be wise to wait until you're about to get a level-up stamina refresh to run it!
Last edited by ekobian 4 years ago ( 9.5.1 )