I've writen two letters to my prevT. The first one I had written as a thank you letter, it was when I stopped with therapy (at the time I thought it would be the last time I would see her).
The second time was when I was back in therapy (after about 1,5 years of no therapy). I had been back in therapy for over a year when she went on leave. Which causes many feelings in me. After a months of her being away I wrote an angry email. It wasn't that long.

On both letter I didn't got a reply. With the first one I had hoped for a short reaction be email, but no. With the second one I also wanted to sort of challenge her.

Oh, and before she went on leave I also wrote her a long email, but that was more about how I felt about everything, how difficult is was for me.
I didn't really get the reaction I hoped for. I was too afraid to ask it straight, and she didn't reacted on all the issues I wrote on, so 2 or 3 important ones where left undiscussed.
But she also could talk around subjects and not just be direct. Mostly on stuff that where about her.

When I wrote things to her, it felt good/relieved. But when I didn't get a reaction back, I just felt hurt.

I haven't broken up with a T by email, thought I've expressed the thought of stopping therapy.