I'm sorry to hear about this, I would never cheat on my girlfriend, back when we were going out, and i knew she would never cheat on me. Maybe you dont trust him as much as you would like too. Think about this, do you trust other people in general? or is this an internal problem? Is there anything in the emails that says "I hope my girlfriend doesnt find out"... or "i shouldnt have done that" etc.?
If you do find out he cheated on you however, you should not take him back, ever.