I have sent a number of emails to my therapist over the past two years telling her that I was upset with her or the way our session went. Telling her that I hated therapy and didn't ever want to come back. Asking her if she even wanted me there and what exactly were we doing that was helping me. I have even written that I hated her, but she knows that it is not 'her' that I hate. It is what she represents and how it makes me feel when I am in a session.
She actually replied to my emails with things like,
"Finally~You are expressing your anger!" and "Good job! Keep them coming!" or "I like that you are able to write down how you are feeling. This helps you work through your feelings. I am not offended by what you are saying. This is what you need to do to heal. Don't worry about my feelings. It is okay for you to say how you feel. This is what we are working for in therapy."
Even though I have worried about crossing boundaries, and have asked her many times about it, not once has she told me that I have crossed a boundary. If that ever happened I would be mortified!