Salutation
If the recipient's name has been used in the inside address, it is usual to use a personal salutation.
Example
Dear Mr. Leighton , Dear Douglas, Dear Mrs.Jackson
If your letter is addressed generally to an organization then the more formal salutaion 'Dear Sirs' should be used.
Example
Dear Sirs
If your letter is addressed to the head of a department or the head of an organization whose name is not known, then it would be more appropriate to use a salutation as shown here.
Example
Dera Sir or Madam