if you're brave enough, sitting down and confronting her about the email might be a strong place to start. otherwise, if the topic comes up, either by you insinuating it or her bringing it up, would be my other recommendation. remind her that "hey did you sign me up for boys soccer yet?" as if her denying you it was never even an option. or if you can't do that either, if she makes a comment about how you're "not a boy".

im all for putting your foot down and standing up for yourself, but i cannot stress enough that if she ever makes you feel like you aren't safe to talk about this with her, to drop the subject and wait on your transition/keep your transition on the downlow until you're safe to do so.