Errant Email Exposes Job Search

My current boss accidentally received an email I sent via gmail to a previous boss at the same company. She was my mentor and I was asking for advice re: an upcoming promotion I wanted to propose to my existing boss. In it I explained that I was looking to move into a more in depth role (my current boss was aware and I had been working in the area already for 8 months) and my 5 year anniversary was on the horizon. In the email I stated my position and mentioned that I'd like at least a title change as other members of my team had all been promoted over the past few years. I also mentioned that if I didn't work out I'd need to rev up my job search. I mentioned a place I recently interviewed at and said I declined the offer. Needless to say my boss went to HR and the next thing I know I am on a “transition” plan to leave in a month as I’m “not happy” at the current company and they “can’t give me the job promotion I want”. I was not brought into the conversation until AFTER they decided the transition. They want me to resign at the end of the month. I”m not particularly interested in that unless they agree - in writing - not to contest my unemployment application. I am a 45 year old female in a male dominated field and team. Do I have any recourse? I have never had a work issue and have been told several times by my current manager that this is NOT a performance issue but “poor fit”.