What she's done in the past is in the past. Whether she's slept with 1 girl or 100 girls doesn't change the fact that she cares about you, and it certainly doesn't effect if she's going to cheat or not.

Your best bet is to always have good communication with your partner. If something they say puts your mind in unease then it's best to talk to your partner about it, because they are the only ones who can help you with the answers that you need to really feel comfortable and secure.

But don't treat your girlfriend like she's on trial. You're not a laywer. Playing this game of "I have proof!" is silly and a bit immature. You should be an adult and discuss and communicate. Not to mention the fact that your girlfriend sent the email to her friend, and not you, but I'm not going to even ask why you are reading your girlfriend's emails...

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So when you're happy (Hurray!), or sad (Aw!),
Or frightened (Eeek!), or mad (Rats!)
An interjection starts a sentence right.

You shouldn't be worried about her cheating on you, just because she has a past with a friend.  If you are worried because of something that someone did when you were not with them, you would have the same problems with everyone.  Just because someone has had sex in the past, doens't mean they will cheat on anyone.  

Just let it go, don't worry so much or you are going to push her away.  I know that your mind is racing, but the best thing to do is wait and see how everything plays out.

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People don't call an Ambulance because they did something right.