I don't like using "/" in formal writing unless it cannot be avoided.
Dear Madam or Sir
Note that it's more traditional to write "Dear Sir or Madam".
If writing to an American recipient, follow it with a colon ) to show respect and formality. A comma (,) is used with family, friends, and other people who are close to you.
Dear Sir or Madam:
Dear Prof. James Smith:
Dear Mom,
Dear Jimmy,
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