I am surprised and disagree that the other answers do not take the relationship and the personal nature of the mail more into account. While I cannot give an answer to your questions, I'm writing this reply to point this fact out in more detail, because I consider it extremely important.
A work relationship is not intimate, you don't have to be friends with your boss and you don't need to trust him with your house key. However, to enable any healthy and productive kind of environment, you need a basic level of trust.
- He trusts you to e.g. do you job (somewhere around) to the best of your abilities and to not be a spy for a competitor (etc.).
- You trust him to not e.g. give you life-threatening tasks or share very personal details with others (etc.).
If I put a request to keep a letter private to someone I have this basic trust relationship with, I expect it to be, in fact, private.
If the above fails to apply, then the environment is bad, and neither side can expect good results. This should be fairly obvious. So, in short, breach of trust is a severe issue, regardless of legal status.
As OP stated, he is working remotely and tried to resolve the problems earlier via phone, finally resorting to email. This can be considered proactive, not unreasonable, as long as you have in mind how your message could be interpreted if the situation were to escalate (court) and word it accordingly. Neither is it unreasonable to assume that basic privacy customs are adhered to, even though, or especially because, he's working remotely.
External influence (e.g. Sony hack) is and should not be accounted for in this regard. If we did that consequently, society would break down. It definitely must be addressed as an important general security issue, but it should have no influence on my decision what to write in an email - unless it is indeed highly confidential, but then you would never write it in a mail anyway, but hand it over personally, probably in non-digital form.