I'm with Mish on this one, I think you need to address the problem asap.
You still have her, she still feels for you and knows shes in a committed relationship with you. Her actions are not excusable, what she has been doing is wrong and emotionally she is cheating. I think that Mark's wife has every right to know whats going on too. Shes in a committed relationship and would want to know just as much as you would if something was going on.
If you let this fester you will lose her or you will fall into an unhappy marriage. Tackle the problem while you have some fresh firepower and before she checks out anymore than she has.
Look, you snooped in her in box, but at the same time you should have never had a reason to, she gave you that. Generally if you think theres something going on, then more than likely there is. You're in a tough spot but you're a smart guy.