"Dear Sir" and "Dear Madam" are used when you don´t know the addressee´s name. Otherwise open your letter with the specific name,"Dear Mr. Miller" or "Dear Mary", depending on the closeness of the relationship. When addressing a woman, Ms. is for those whose marital you don´t know or don´t want to specify with Mrs. or Miss. Many women prefer Ms. as a non-sexist form. Note that in British English, spelling without the full stop - Ms, Mrs, Mr - is standard. When writing to at least two men "Dear Sirs" or just "Gentlemen" would be the right salutation. The first is preferred in Great Britain the latter in the US. While such salutations also encompass female readers, they may be deemed old-fashioned. Therefore "Dear Sir/Madam" is often preferred. After the body of the letter the writer has to find a suitable ending. The complimentary close must fit the salutation. Formal letters starting with "Dear Sir" or "Dear Madam" would end appropriately with "Yours faithfully". The matching ending for "Dear Mr. Miller" would be "Yours sincerely" or "Sincerely yours", the latter typically American English. "Yours very truly" and "Truly yours" are also pretty common in American English closes. Less formal endings are "With best regards", "Regards" or "Best wishes".
Learning English: Start and ending of a business letter
When writing an English letter, the first hurdle is to select the appropriate salutation.