First name or last name with "Sir"

If my teacher's first name is Robert and his last name is Dowry, and I have to send him an email, then which of the following will be correct?

  1. Dear Sir Dowry,
  2. Dear Sir Robert,
  3. Dear Sir Robert Dowry,

Solution 1:

If he is knighted, it would be Dear Sir Robert,

Otherwise you would use any of

  • Dear Sir,
  • Dear Mr. Dowry,

or if you know him well or are American,

  • Dear Robert,