Dear Confused in Texas,
You Can't Change Others
The one thing that I know of people and of myself is that I cannot change another person. I can only change myself. So on that premise, I'm going to respond to your question with how I would respond to that situation.
Invasion of Privacy
Your mother in law invaded your privacy by listening to your voice mail and passing on private information that you shared with her to her friends and colleagues.
But I might add that the emails that you found were in her house, according to the additional information you provided the panelists. You ultimately had no right to open the folder that you found them in. You, in essence, invaded her privacy as well by doing this. Perhaps it's best to practice what you preach, and if you expect someone to respect your sense of privacy, it's important that you respect theirs.
Two Wrongs Don't Make a Right
No matter what was written in those emails, you ultimately had no right to read them without her express permission just as she had no right to go through your voice mail. Do two wrongs make a right?
Decide What You Want from Her
You asked ultimately if you should confront your mother-in-law on what you found. First, you should ask yourself what you hope to gain from doing that. Deciding what it is that you want as a result out of this should help you decide whether or not to confront her.
What is it that you are looking for? Do you just want to tell her that you know? Why? Do you want an apology? Do you want her behavior to change? Do you want her opinions and attitudes towards you to change??
I think it's very important to decide what outcome you are looking for, decide whether or not you can actually achieve that outcome, before you take this face to face with your mother in law. If you aren't clear from the onset what you want from her, it might be very difficult to get it.
It's almost impossible to change someone else's behavior. It is, however, quite possible to change your own. Unfortunately, you have seen evidence of how your mother-in-law feels. You do know what to expect from her, and given that, rather than expecting her to change look at how you can possibly protect yourself and your husband from further problems with her.
Obviously, change your password on your voice mail that is always something you should do anyway, because not all people are trustworthy.
Distance Yourself from Her
But more important is to realize that you possibly cannot trust her with your personal information, with your feelings. I would think it would be important to find a way to distance yourself from your mother-in-law. You can't change her attitudes or opinions about what is acceptable behavior, but you can keep yourself from getting into this situation again.