I have heard Xin chào! is more like an archaism now. Would you rather use just chào alone nowadays?
Xin chào sounds a bit more formal than just Chào. In everyday conversation, use can say Chào to your friends but it's recommended to say Xin chào to a complete stranger or someone who is older than you. Even TV presenters/show host say Xin chào most of the time.
Depends on who receives your email. See the answers above for more info. :D