Welcome to the forum.
A formal letter to a man starts "Dear Sir". However, "Sir" is also a formal title in the UK for someone who has been knighted. Are you writing to someone who has been knighted by the Queen?
Remember to end every sentence you write with a punctuation mark. I note that your member profile says you are a native English speaker. As such, it's important to use correctly written English at all times on this forum. Learners of English will assume that everything you write is correct when they see your profile.