Jeff, I feel for you. I had almost the exact same experience with finding my H's e-mail left open by accident, and not being able to resist snooping, and finding proof of a PA that I had strongly suspected but was hoping was all smoke and no fire. Yes, it is very painful to read their erotic e-mails to each other. The only difference is that H caught me reading his e-mail, although he wouldn't have known which messages I looked at (there were tons of them--the EA had been going on almost a year at this point, although the PA was considerably more recent).
I would wait on confrontation if I were in your position. (My H and I have never discussed his PA and I don't even think he is aware that I know about it for sure.) This is like squeezing the toothpaste tube--very difficult to reverse. If you approach it badly, things will be said that you can't erase, and it will be a lot more work to recover.
You may decide to confront her later, but wait a while, until you can decide on a course of action without your emotions skewing your logic compass. When in doubt, do nothing, except to seek and wait for answers from whatever you consider your higher power (or your subconscious, if you prefer).
In the meantime, breathe through the pain until it lessens, and find healthy ways to relieve the stress.
Peace and blessings, Dawn
Me 45/H 47, no kids Together since 1985; M/1992 Bomb1 (EA-OW1, age 22) 2001 Bomb2 (EA/PA-OW2, age 22) 10/2007, A continues H left 11/24/08 minimal contact, no legal action http://tinyurl.com/DawnHope1