There are several ways to address a letter to an unknown person, including: "Dear Sir or Madam," "To whom it may concern" or "Dear Sir." Not knowing the gender of the recipient of the letter makes "To whom it may concern" the most practical choice.
The salutation on a business letter should be formal, whether or not the author of the letter knows the person. If the author does not know the gender of the recipient, it is possible to use the job title in a salutation.
Once the letter's author has the salutation figured out, he or she can begin constructing the letter. The sender's address should be at the top of the letter, when a formal letterhead is not being used. This is followed by the date line and the recipient's address. Follow the inside address with the formal salutation, noting any title the recipient may use such as "Ms.," "Mr." or "Dr." Everything on a business letter should be left-justified.
Follow the salutation with the body of the letter. Then, end the letter with a closing paragraph and note any enclosures included with the letter. Enclosures include things like a resume or other document. If someone else typed the letter, the typists initials are included at the bottom.