I know your need to send her an e-mail but if you're expectingf her to sympathise with you, forget it.
On d-day I posted something like two-faced bitch on her fb page. Her fb was rapidly taken down.
Wrote 2 visceral, brief e-mails to her, informing her that I knew what was going on and her H now knew too. Also told her any bold c*** would have done (true).
Several days later I received an insulting, hypocritical,grovelling e-mail from her saying she 'never meant for any of it to happen' and signed off with'all my heart'....you get the drift, trying to minimise damage and cover her arse. Her husband couldn't believe the cheek of it when I showed him. Well it had the opposite effect,made my blood boil and I let rip to her in a long e-mail, told her exactly what I thought of her which wasn't pretty. Also indicated that my rage at her was out of control, and whilst not actively seeking her out I wouldn't be responsible for my actions if I ever saw her. It had the desired effect, it gave me a chance to vent my feelings, gave me a chance to tell her a few home truths which I'd been too polite to tell her in the past(I'd known her for years), and apparently left her cowering in her house and rented appartment in case I caught sight of her. Yes it was a pretty vindictive e-mail but I don't regret any of it, sent a copy to my H and her H, and at least I felt there were finally some consequences from her actions as she was completely unremorseful both during and after the LTA.
Finally, nearly a year after d-day, and once I'd made sure her H would be ok financially whatever happened between them, I e-mailed her work and university where she was doing a phD sending copies of the some of the juicier e-mails to show how she'd used work time, office space, work e-mail to continue the A and download large quantities of porn (strictly against their code of conduct). Don't know the effect this had on her job or studies, I actually don't care, but I know she was called to book over it and I love imagining how she would squirm!
So sending an e-mail can be very cathartic, but don't expect her to see anything from your point of view, don't prostrate yourself and your hurt in front of her, as others have said she really couldn't care less. But sending the right e-mail to the right person can feel really good. No regrets on my part.