Think of it as whatever past experiences he has is what you like in him or those are what made him what he is today. Don't dwell on it and definitely stop asking him. He is with you and that's what matters. If he is still showing them affection or whatever then be concerned.
I understand how you feel and how hard it is not to think about it every time you see his face.. But the fact is.. If you value this relationship you have to let it go like you know he would if it was him in your place.
You don't even deserve to be with this man if you're going through his personal emails. He loves you now and you don't have any reason to think that he doesn't love you. You need to grow up. I assume you're a lot younger than him or something.
There are chances that your boyfriend is joking to tease you. Why you ask such questions which depress you. Try to understand his nature and habits without asking such questions. When you understand him, then it will be easy for you to know about his past. If he is sincere with you regardless of his past and you both have mutual understanding then keep continue.
Oh man that just happened to me yesterday! I actually just got married and I was looking in his email account and I saw emails from 2 years ago saying he loves her and stuff. The best thing to do is to move forward because you are with him now. I hope I can get over it fast cause its killing me! Good luck
If your so jealous of his past then the best thing for you to do is leave his past in the past, Why ask ? Realize its you n him ,, Go forward..