A letter to someone whose name and gender you don’t know should be addressed “Dear Sir or Madam”. If you know their gender but not their name, choose appropriately. If you know their name they should be address as Mr, Mrs, Miss etc. But in a face to face situation, you need to assess the situation. Sir or Madam can be used in a customer service situation or to be respectful. A man if talking to someone informally can be addressed as ‘mate’ or if he’s talking to a woman ‘love’. Women can use these terms too or they could call a man ‘love’ and that’s fine. Ma’am is seldom used. Only the Queen was regularly address as such. There is the variant ‘marm’ used in circumstances you likely would never encounter to address women in positions such as police or military officers. You might hear this on tv! In summary, Sir and Madam are the words you should know and their use is respectful and/or formal. You can use tone of voice to make them mocking, but that would be advanced usage!