email - 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,

Answer

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,