Be careful when greeting across cultures. Many years ago, I was learning German. My prof told me that "Wie gehts?" was replacing the more formal "Wie geht es Ihnen?", especially among close acquaintances. Soon after, I happened to visit my "uncle" (really, a close family friend), who emigrated to the U.S. from Germany as a teen soon after WWII. I greeted him with a jaunty "Wie gehts?", hoping to impress him with my slick German. He was mortified! After growling something in German, he informed me that "Wie gehts?" was a greeting that only very low-class Germans would use, and that using it with one's uncle was insulting. (Ach, those class-conscious Germans!)
My point is, even when you think you know the culture, I would err on the side of formality for a first email. Your greetings can gradually get less formal as you trade correspondence.