So her reaction was kind of what was expected but at least I've got it off my chest.
Good. I was afraid of it being softened even more. If you can't tell your wife her behavior is wrong, hurtful towards her son and she's being a bad parent right now, than who can?
IMO, this is part of detaching. You sent an email directly confrontational to her knowing it would upset her but no longer allowing her anger to make you fearful of stating your feelings on the issue. Fear doesn't control you anymore. It's liberating to assume the position that I don't care if you are mad. You aren't divorcing me because I made you mad...you are divorcing me because you have stronger inappropriate feelings for an interloping character lacking man and I'm not going to sit here anymore and pretend it's ok.
The letter thing IS scary and it's attractive to the BH's because we men lack the emotional experience to express our feelings confidently. So we write letters that we can revise to say exactly what we want them to say. We also, as I've mentioned previously, try to manipulate and play this game "for the win". That's why all the initial "i love you" and "I've changed" letters are so often drafted and sometimes actually sent under the mistaken belief that we can fix our wives ...."if they only knew".
This letter is different. But spending a day on it was kind of overkill. You kind of have to be ready to respond quickly all on your own to such an outrageous email even if to just ask the question "So, let me get this straight, you actually care what's in our son's best interest?". THAT is a conversation opener. She'll spew. She won't like talking about being a bad parent because she already knows she's being a bad parent (way wards are delusional but even they know that texting 1000 texts a week kind of cuts into their parenting time). You didn't do anything other than ask a question and let her convict herself and calmly listen.
You are now detached from fear.
The internet is 90% complaining and entitlement and I hate it because I deserve better!