OK; IF you do regular back ups you may be able to: clone your current system (SuperDuper), then set the computers clock back to the date of the last back-up, restore to that back-up, recover your junk mail, Reset the date & time and restore the original.
Is it worth it? Probably not.
BTW: Junk boxes and trash bins should not be used as interim storage. Create a folder for those needs. I created a "PreTrash" for files I wish to review before I delete them.
One further comment resetting the clock in OSX is always a very bad idea. If someone else can think of a way to change the "delete after one week" setting before you open the mail application I would go that route instead.