Time ago, Señora was used if the woman was married, and señorita otherwise.
Now it's more complicate, but saying señora you go well for sure.
Saying señorita it's not so bad if you both are on good terms, she will feel younger and happy, but it's a formal one you must say Señora.
suposse it's Miss Sue Wormwood
goes from normal to casual. I don't really know how close
Muy estimada señorita/señora Wormwood.
Apreciada Señora Wormwood <-- best one, middle one,
Querida Señora Wormwood
Querida Sue
Hola Sue
If it's a former profesor, and you are on good terms, you can say
Querida/Apreciada Señora Wormwood,
le escribo estas letras para agradecerleo sinceramente el esfuerzo y paciencia que dedicó para que yo aprendiese castellano correctamente. .....
Saludos
