How do you address a Lady when writing?

* If you have just met this lady and do not know her well you can still call her by her first name or, if you are dating you could say 'Dear Jane' and continue on from there. If you both are new to the relationship then sign it 'Fondly Yours' and sign it with your first name only. If this lady is just an acquaintance and elderly then you should address her as: Dear Mrs. Doe