that should not happen for which old emails that you sure you deleted "magically" reappared in inbox due to computer crash fix.
but are you using some windows live or outlook program?
but if you are using something like a ISP's email system, ISP's might keep copies of your old emails. NOT for soliciting nor do you have to worry about privacy, BUT if you were to get into any legal issues requiring email check.
Other than that, i don't know why you get very very very old mail back if you certain it's gone the first time.