My other reaction is please make sure D17 has access to help and resources. She may really benefit from having somebody to talk to privately about her feelings that she has bottled up for so long.
Both daughters see a therapist once, if not twice a month. I know the younger daughter talked about this most recent incident (seeing the email) with her therapist the same day she spoke to me about it but I don't know if she ever talked about the A with her therapist before that. As far as I know, my older daughter does not know (she is 19).
Me: 48 y/o W: 47 y/o Together: > 20 yrs BD: Dec '15, then S 2nd BD: Mar '16, then I filed for D April '16: started piecing