Sir
noun [ S ]
formal uk/sɜːr/ usa formal and polite way of speaking to a man:
May I help you, Sir?
Dear Sir
the usual way of beginning a formal letter to a man whose name you do not know:
Dear Sir, I am writing to complain about the quality of the service I received last week.
Dear Sir or Madam
used to begin a formal letter when you do not know whether the person you are writing to is a man or a woman:
Dear Sir or Madam, I am writing to inform you that I wish to cancel my credit card.
Dear Sirs old-fashioned
used to begin a letter to a company when you do not have someone's name to write to directly
Compare
Madam