As with so much at that time, it depends!
A lady (and gentlemen assume that all women are ladies, no matter what their income) is addressed as "Miss" or "Mrs." and her last name. A proper introduction means you have the privilege of addressing her at all. Men didn't speak to women they were not acquainted with, except in time of dire need. "Madam, mind the runaway omnibus approaching!!"
In public, a gentleman would address even his fiancee as "Miss" plus her last name, except in the South. Being a doughty Unionist, I don't know the nuances of when one might address Miss Susan Smith as Miss Susan or Miss Susie or Sue or My Little Sugar Pie, south of the Mason-Dixon. In private, among friends and famly, people addressed each other in variously familiar forms.
Those of the "lower classes" were decidedly more familiar with each other.
Letters are private documents, meant for the eyes of the recipient only, and so first names and even pet names might be used. A man writing to a woman whom he has not met, or just barely met, is being bold and even fast by using her first name.
Anyone who's interested in the subject should take a look at this website:
http://www.ladiesofreenacting.com/Victorian.html
Much good information!
Zou