Unless you had an advance agreement with them before you sent it, or they were in a position of professional trust that was expected (i.e.: a business associate, accountant, etc) I don;t think you can impose a duty of confidentiality upon them just by demanding it.
That said, if you sent it to them with a stated expectation that they would not disclose it to others, and they did so,
and you suffered financially tangible losses you
may have some sort of cause of action to bring a lawsuit against them. Lots would depend on the nature of your loss and their intention when they passed it on.
ie: If you told a friend that you trusted that you were thinking of doing some sort of business deal and you asked for their advice and they went and they passed that information on to someone with the intention of adversely affecting your business or you deal, and it did, then you
may have some sort of action based on
"tortious interference".
But if you wrote to your buddy and bragged about how you were banging your secretary because your wife is a botch, and he passed that on to your wife, or his wife or buddy and somehow it got back to your wife and you got divorced and had to dived your family assets, pay support etc... or you wrote to say what an asshole your boss is and the guy passed it on to another employee etc and you got fired, then you have no one but yourself to blame.