How do you address in a formal email when you only know their first names? I emailed a new business partner not knowing who he/she is, so I first addressed in email "Dear Sir or Madam,". Then that person replied and ended his email with only first name (let's say, John). Now I need to reply again and I will have to be very formal because this is very important as I asked for his favor. How do I address not using "Dear John," because he's much older with the position and I'm still quite young. He did however only addressed to me by only first name. If I put "Dear Mr. John," it wouldn't be correct because "Mr." is followed by last name. Or just put "Dear Sir,"? "Dear John,"? Please help.