In Mary's personal letters....

[eperon]

In reading the wonderful book that comprises of all of Mary's correspondence over her lifetime, I noticed that even in her intimate letters to her dearest friends, e.g., Sally Orne, she never opens the letter with "Dear Sally..." It was always the more formal "Dear Mrs. Orne..."
This must have been the convention of the era, but it seems so stiff to our 21st-century familiarity.
Does anyone know about the letter-writing conventions of the day??

Thank you!!