"Dear you" is a bit awkward. It is uncommon to address a message to "you", because the word "you" changes to everybody person reading it. See, when I read this "you" means me, but if my mom were to read this message "you" would mean her, and so on. Who you're trying to send this message to has to remain the same. Using "you" is often used in more informal conversations. Also, I have no idea what message you're trying to convey with "If I am up to the mustard".