Im so sorry you recieved that email. I know that it had to hurt.
I am not so sure that I would have sent one back -for a few reasons.
First, that is exactly what whomever it was hoped you'd do.
And you dont want to give any power to that person, even if it was the ow.
You want to show that she is nothing, not worth your head space because it's true. You dont want to give credence to something that is built on lies and deceit.
Although I am glad that you reacted calmly.
Whether your h is in mlc or not, your actions are the same. Give him space, move forward, do the work, look within, make any changes and buckle up.
I know you said your marriage was near perfect, but, I think if you really think about it, there are some things that could be worked on, right?