From Longman Dictionary of Contemporary EnglishDear MadamDear MadamformalTCM used at the beginning of a business letter to a woman, when you do not know her name → madam
Examples from the Corpus
Dear Madam• It is a house of harmony to my hand. Dear madam!• One was considerably shorter than the other. Dear Madam, Since our extraordinary conversation I have thought of nothing else. 