If you are writing a formal letter don't put the person's name in the salutation; instead put it in the header. Use dear sir or dear madam . If you don't know the name or the sex of the person you wish to contact take the trouble to

It is not 'Ladies' simply because we do not use that in a business salutation; there is no particular logic to it, as far as I know. Are the 'ladies' and 'gentlemen' below different from each other?-- 'Ladies and Gentlemen, the next President

In the salutation, you should use their name, e.g. Dear Mrs. and Ms. Smith: 'Mesdames' is plural for 'Mrs.', and 'Ladies' is wrong for direct address in a letter.