Hi - you have quite the dilemma here. As you said, reading her emails was the wrong thing to do and now you have the advantage of being privy to her past relationship. Regarding the above statement - some people can do the 'friends with benefits' thing and that doesn't mean your GF was taken advantage of - seems like their relationship was for sexual convenience.

You happen to have high moral guidelines but she may have her own but not necessarily wrong ones. You admit you're nosey, so now you're judging her based on her past and knowing how she interacted with this man. I'm wondering how can you avoid being overly suspicious given what you know? Has she told you about this guy? Are they normally friends without the sex, when she normally has a partner? Also do you feel guilty for snooping and isn't that an act of being untrustworthy on your part?? I'm not picking on you, but trying to help you see that snooping proves, you can hide info and feelings in the relationship.

Do you think you're going to continue being suspicious based on what you know? Trust is one of the main pillars of a relationship - she hasn't broke that trust but you have. I hope you'll be able to enjoy the present with this woman and not let your knowledge of the past, along with your mistrust ruin it.