Oh, dear... I am very disheartened by this.
Back in my Christian phase, I read the entire New Testament and about half of the Old Testament. I diligently studied every text and interpretation I could find even remotely regarding ->-bleeped-<-. There is one passage that talks about how a "man shouldn't wear women's clothing, or a woman wear men's clothing," but it's an incomplete translation. The original text says "a man shouldn't wear women's clothing to enter a tent and a women shouldn't wear men's clothing and cut her hair to join the army" (rough paraphrase there). That's about it. I assume that your mom's fears manifesting themselves as pure vitriol she's basing on her religion? Many Christians don't bother learning why they believe what they believe, but they just know they feel wrong about it, so it MUST be God telling them it's wrong. This isn't the view of all Christians, but I've encountered enough to recognize the behavior.
If you're able and willing, maybe you could try to have a calm, honest conversation with her about why she felt it was necessary to write the note. If she tries to bring religion into it, maybe you could ask what passages she's interpreting this from or ask her for some time to do your own research. I think for me, I'd lack the amount of patience it would take to try to be the rational one, but if you can muster it, try. It could be that she's full of hate, in which case, leave with no regrets. However, if her reaction and that letter were based on fear, uncertainty, misguided faith, or a belief that she had some other plan for your life, you may not want to throw away your entire relationship with her.
I don't know you or your family situation, but I truly wish you the best. I'm estranged from my family because of unresolved issues with being physically abused by my father as a child, but I really wish that I hadn't cut myself off emotionally from them. Angry words and arguments can be forgotten as long as you stay around to heal together, but walking away just cauterizes the wound and leaves you with scars that will never heal.