Dear Sir or Madam is ok. But if you can get to know his name, that would be better. Also, be very careful about the spelling of his/her name. It shows you've exerted effort to know the details.

True, first impression lasts, and it applies on the appearance of the letter. Remember that it is not you writing the letter, it is the company. Whatever comes from your company, is from your Company.

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Everything happens for a
reason.