I am not regretting reading the e-mails, I am actually relieved to know what she is saying, but the lady is spitting venom and has created issues since we were married. How do I move on from here?
Answer: Please know that since I don't know you, this is not personal:
You are a control freak.
Here are the dots I connected: You have set ways at home and expect others to conform; this is so integral to your worldview that you assume other homes come with rules, when maybe others have looser standards; you regard playing by house rules not as a courtesy so much as a quid pro quo; and you're deeply invested in what others think of you, thus your suspicions and your willingness to violate your mother-in-law's privacy - without regret - to confirm those suspicions.
It is a hallmark of controlling people to be preoccupied with the way they appear to others, even to the point of trying to shape people's perceptions vs. just letting them think what they're going to think.
In general, it's helpful to see this not as a set of actions, but instead a set of re-actions. Wanting to have everything just so is often a reaction to a fear of chaos, or perceived helplessness against external forces.
So the first step in dealing with your mother-in-law is to assess the way you deal with the world in general: Ask yourself why you're fussy, why the house rules, why you need to know what others say about you.
Understanding the source of your unease is point A. Point B is where you trust yourself enough to believe you'll be OK, even if you're less than vigilant where you used to be "particular." Therapy or not is your call, but it can be very useful to have good third-party guidance in getting from A to B.
The next step is mother-in-law specific: Make a generous peace overture. Along the lines of "I realize I have really dug in on some things, and I regret we got off to the start we did. I'm sorry."
This will go much better if it has a foundation of genuine forgiveness. Consider that her "nasty lies" probably start as truths, which she then bends to form the narrative she needs - not to tear you down so much as to build herself up. Another hunch, but, which type of person do you run across more: evil, or scared?
E-mail Carolyn Hax at firstname.lastname@example.org, or chat with her online at noon Fridays at www.washingtonpost.com.