I rarely disagree with our esteemed Cas, but I have to throw in my two cents here....I was taught years ago that the proper salutation for a business letter if you don't know the addressee's surname or sex is "Dear Sir or Madam." It sounds antiquated, but it is still the accepted form. Even the venerable Miss Manners agrees:
NewStandard: 9/18/97
...That tells Miss Manners that there is an urgent need for a set form that people can refuse to use because they feel that it is too formal or too cute or not original. This will inspire them to improvise something original that will be off-putting to their correspondents, who will consider it too formal or too cute and not even all that original.
Isn't that the way we now do it with snail mail?
Speaking of which, how is it possible that there are still people alive on earth who are astonished to discover that ladies are now sometimes permitted to open mail at business establishments, and that therefore the salutation "Dear Sir" to an unknown individual may be in need of revision?
Miss Manners receives this startled cry by every post, even though it is more than a century since gentlemen came up with the not-altogether-altruistic notion of unloading the office mail on ladies.
The answer is "Dear Madam or Sir." In the plural, it is "Ladies and Gentlemen," although Miss Manners is willing to let pass the less traditional "Gentlewomen and Gentlemen" for those who still believe that equality is best symbolized by making female conventions look as much as possible like male ones. (If the terms must match, why does no one ever suggest "Ladies and Laddies"?)
Yes, these are formal modes of address. Do you really think it wise to get smart with someone whose gender you don't know?
Miss Manners considers it her duty to repeat these business-letter forms as often as necessary.
Proper salutation 
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