I was about to send email to U.S. today. As it was a very important email, I asked a Australian writer to proofread it. She changed the first name addressing to Mr. XX saying that it's better that way if I have not seen him before. I said to her it is common to call by first name especially in U.S But she was not satsfied. So I asked another Canadian writer. He told me unless I know that he wants me to call him by his first name, it would be safer.