following is two lists of salutation - one to a man and the other to a women. Both lists begin with the most formal and descend to the least formal.
"Sir" is used in writing to person high government position ant not in ordinary business letters. The most commonly used salutation for a business letter is...
When writing to a firm or an organization, the appropriate salutation is "Gentlemen:" for U.S.A.
if you are sure you are writing to women, use "Ladies: