I would reply with something along the lines of "W, you sound angry and frustrated, I'm sorry you feel that way. I would prefer that we stay together and work on the marriage, but I do want you to be happy and if you feel that you must leave to be happy then I support you in your decision."
This is similar to what I told my W (in person) when she talked about leaving. It is based on a recommendation Charles Dobson made in his book "Love Must Be Tough".
I'm sure you're tempted to defend yourself, but don't do that as it will just anger her. For example, she says "Sticking your head in the sand and pretending everything is OK". I'm sure you want to respond by saying you do NOT think everything is OK, far from it in fact. But that is just arguing and arguing is pressure. Besides, she's just spewing. She knows darned well you don't think everything is OK.
