Wow, what a situation. Truth be told, I have never had a girl friend (and I dont mind that fact, girls are too expensive/stressful) so I dont know exactly how I would feel in this situation, but I can do my best. Also, somethings may come off horribly blunt and I am sorry for that, they are not meant to offend whatsoever.
First off, who the hell are you to tell her who she can be friends with and who she cant? Shes a big girl and you arent her master, I think if she wants to talk to her ex thats her choice, not yours. You've got to trust her, trust is an integral part of any relationship. However, she violoated that trust in kissing him and lieing about being in contact with him. If a girl violates my trust, her ass would be on the curb without question.
Secondly, she is not your master and who the hell is she to keep you from your friends and band? Just as you have to trust her, she has to trust that you will remain faithful and give her the love and attention she deserves as your significant other.
Thirdly, though its nice to be in the know no matter what, going through other peoples mail is not cool. Would you be pissed if she went through yours? If you do/did find out something, I would definately not call her out on it directly, that would be pointing out to her that you do not trust her and she would in turn not trust you.
So heres what I think. You two need a break or a serious heart to heart talk. You both need to be more open with each other and be willing to accept each other a bit better. Shes got to take the leash off of you and let you be with your friends, but you have to trust that she isnt going to cheat on you. If she does, drop it like its hot.