What can I say. This is a bit shocking, Jason.
How do you think she will react? How does she feel about homosexuality? Why an email? Do you think it will be cathartic, regardless of her reaction?
Best of luck, Jason.
She's OK with homosexuality and so is my step-father -- at least in other people. They have gay friends in their WASPy social circle (cocktails, horses, and golf) where it's cosmopolitan and expected to accept gays at least in some measure.
The difficult part is that we do not discuss sex. EVER. Not when I was growing-up, not ever. Both my parents would say, "Ask us about anything," but any attempt to ask was met with, "Perhaps you should speak to Dr. K or your father." My father was useless as I never felt comfortable discussing anything personal with him and in my teen years I came to distrust both my parents with anything too personal out of fear they'd use it against me.I'm writing a really long post about growing up and all. My mom is very down on men. She went through a bad divorce with my father and if she can't control men around her (me included) then she doesn't trust them. We're cordial to each other but not necessarily warm. She's never mentioned girlfriends or boyfriends, never asked about grandkids or bringing someone to family things. When I was very young I tried bringing these up but they were met with stiff silences which essentially told me that the topic was not welcome. Meanwhile my sister was encouraged to date, take care of herself, and find a good man with prospects however, my sister was also encouraged to get her master's degree and have an independent career so she would never be dependent upon a man. My mom's definitely an Emma Willard girl. No question about it.
Email is good for me because I'm a good writer and it will be easier to deal with an aftermath than with exposing myself emotionally in person. I can't be goaded to say something I'll regret or have my buttons pushed or have the conversation devolve into something else entirely. She is also a very good writer and can plan what she will say without letting her emotions dictate them. If she has to have a crisis or ask a barrage of questions, I can handle them better in print and plan my replies.
I don't think it will be horrible but nor do I think it will be easy. My mom is very unpredictable about these things though I think she will be the easier of my parents to discuss this with.
You are 42. If you are also single she may not be in total shock.

But good luck and hope all goes well
I'm quite sure though I did pre-emptively deny it during a family therapy session when I was a teenager. She will remember that and wonder why I lied to her. I have to tell her I simply was not prepared to admit to myself much less anyone else or particularly my parents with whom I did not get along with at all during that time.