This is where it gets tricky. My ex (the abusive guy) used to cheat on me too. Granted, it was a high school relationship, but his actions were past the high school point, if you get my meaning. He would lie about cheating, and only come clean if I found out from someone else. It progressively got worse over the years.

I don't want to say, break up with her, or set an ultimatum like "if you don't stop talking to him then we are through". But I understand your distress completely. I know it is hard to trust someone that you fear is being dishonest with you..

Perhaps you should ask her if she has been talking to him, and ask her to be honest. If she tries to lie, I think you have your answer about what to do. Is the date on the e-mail recent? Maybe it is hard for her to cut ties with him for some reason and it COULD be harmless, but I would not want my ex to talk to the girls he cheated on me with, he would, and it hurt, and I could never trust him..or them. It led to destructive social behaviors. It led to me distrusting almost anyone that was not my friend, and sometimes even my friends. Granted, he cheated with A LOT of people, so it was a bit different. I think you should talk to her. Maybe not immediately confront her about the e-mail. I believe it will make her angry that you snooped in her e-mail.

Also, the fact that you felt the need to snoop is bad. I know the feeling, but you can never truly be happy if you always have to wonder if she is being honest with you. It would be different if it were a thing that happened a while ago, and she dropped it. My ex tries to talk to me ALL the time, it drives me nuts, but I always tell him that it's not fair to me or my current boyfriend that we talk. You should tell her that it hurts you that she feels she has to lie to you. Tell her that it also hurts you that you have to wonder if she is being honest, and that you don't want to be hurt again..

I think I'm rambling. Basically, try and talk to her and maybe see where things are going. My ex and I dated for close to 3 years, so it is possible to let go of a liar (if she really is one) and find someone that will treat you fairly and honestly.