Leave communicating with the ex to your husband. And perhaps he should try to limit it to writing. It gives both sides a better chance to think over what to say. And if someone does get upset and go off half-cocked in an email, at least in the midst of all the emotion, no one can be accused of saying something they didn't say; or failing to share information they were supposed to share.
Naturally, it upset you to be insulted as you were. Naturally, it upsets your SD to feel she was born in the wrong gender; to have grown bold enough to say so publicly; and to think it disgusts and upsets her mother.
Naturally it is ALSO distressing for a mother to be separated from her child; to be told that's the way her child wants it; and to have that statement come a woman Mom feels in competition with - a competition she's losing! You have replaced Mom, in your husband's life. You seem to have replaced her in the child's life, too. And if Mom calls you "trophy wife", it's because she thinks you're younger and prettier than she is, on top of everything else. Imagine for a second how all of that feels, on her end.
I won't defend the things Mom said to you. But many people say angry things when they're angry. To fault her because she stopped being sugary-sweet after you told her the child she carried in her womb wants to "leave her in the past" is as unreasonable as having a friendly chat with a cop, then complaining when he puts you in handcuffs after you pleasantly mention the illegal drugs in your car! It's not fair tell a person something that's designed to upset/devastate/infuriate ANYONE in their position; and then say, "What an angry person! See why I don't want her around?"
Maybe Mom is an all-around angry person. Maybe there are good reasons for her child to stay away from her. I'm just not seeing them, in your post.
> She was strict? I know some stellar parents who were strict. Their kids complained, while they were kids. But now those kids are productive, well-adjusted adults with good educations, good jobs, and the discipline to chase their dreams even when it's hard, exhausting or discouraging. They have deep respect and gratitude for their strict parents. Maybe it's different with your DSD's mom, but strict - IN ITSELF - is not evil.
> She's "transphobic"? No doubt, any child navigating the shark-filled waters of gender uncertainty should have someone in their lives who will be only loving, supportive and positive. That must be a hard enough journey, without feeling all alone! So it's lucky DSD has you. But these days many people pat themselves on the backs for being politically correct and progressive enough to show sympathy, acceptance and non-judgmentalism toward historically picked-on groups (like homosexuals or transgender). What about showing "sympathy, acceptance and non-judgmentalism" toward people who TRULY have different world-views from you own...like DSD's mom? Trying to understand her perspective doesn't mean changing course and agreeing with her.
Although you don't agree with Mom, can't you put yourself in her shoes a little?
>>> She may have been raised since earliest childhood to believe homosexuality or changing genders is "wrong", or offensive to God. She may think trying to change the gender God or nature gave you is just as "wrong" as YOU think "transphobia" is.
>>> ANY parent who thinks their child is on the wrong path hopes to guide them back to the right one. If one of your bio daughters started saying insulting things about gays, would you be open and accepting of her different view and try to embrace it with her? Or would you counsel her on what you think is correct; and try to keep her away from bad influences? It's likely the same, with DSD's mom! She may think the loving thing, as a parent, is to look for some way to help her son get back on what she believes is the right path; and that accepting a gender change encourages and enables him to continue doing something wrong.
>>> When a child does something a parent sees as "wrong", or when a child rejects a parent, the parent naturally wants to believe it's the fault of outside influences. ANY parent wants to believe their child is inherently good and - left to his/her own devices - will love the parent, want to continue a relationship with the parent, care what the parent thinks, and share the parent's values. Don't you want to believe that about your bio kids? Even if she's wrong, it is understandable that your DSD's Mom wants to blame you (and your DH, I assume) for turning DSD against her, and against her values. That's a way for Mom to acknowledge behavior on DSD's part that Mom finds unacceptable, but to avoid blaming or faulting DSD for it (by faulting you, instead). Convoluted as it is, it's a way of being loving. Maybe the gender identity issue could divide you and her, instead of dividing DSD and her?
Clearly, it sounds like Mom's emotions get in the way, when she has difficult situations to handle - be it a phone call with you, or DSD coming out. Don't you think it upsets her, too; that she can't figure out a way to handle DSD's gender identity issues without driving away her child? But this isn't a parent who found out her son wanted to be a girl and decided, "You're dead to me forever." This is a mother who still wants to be in her child's life. That's why she keeps calling, even if she can't figure out a way for any of those calls to go well.
It's easy to be the hero to DSD - to be the mother she "really needed" and to stand by her while she rejects and hates her Mom. But counseling - or occasional supervised visits (where DSD has some support, if Mom gets upset) - could eventually bring DSD and Mom together, a bit. Cutting off contact forever cannot possibly ever accomplish that.
The more challenging role you could play would be to help DSD understand how hard the gender issues are, for her Mom - and why. It's not nearly as simple as Mom hating her. If Mom hated her, she'd quit trying to have contact. Deep down, no kid wants to believe the woman who gave them life quit caring about them, or quit wanting a relationship. And that is NOT the case, in DSD's life. Her Mom just needs help - maybe a lot of help - figuring out how to conduct that relationship.