Before greetings, you should include subject of matter to give your reader a clear idea about the letter. Addressing your receiver with “Dear” is a common practice in the business world and it is followed by first or last name. It is better to use first and last name as there is no personal relation involved in business correspondence.
Do not mention marital status of your recipient unless you are sure about that. If your receiver is married then you will use “Mrs” before the last name. If she is single then “Ms.” Is an acceptable title.
Sometimes you may find yourself in a situation when you do not know her marital status. In such kind of confusions, you should play safe and address her as “Ms.” Followed by her last name.
If you do not know the name of your addressee then you should go for a general title like “Dear Madam”.
You may use her professional title/designation followed by the last name. If you are not sure about the gender or marital status of your addressee then this practice saves you from any awkward situation. For example, if you are writing to a doctor then you can start with “Dear Dr. Jones”. However, do not use a lesser title as it will create an unprofessional image of yours.
In case of any close relation, you may enjoy the liberty of addressing her with first name or in bit informal way. For example if you are writing business letter to a very good friend then you can open your letter with “Dear Sara Parker”.