In the 17th century writing letters was something men ought to do. The practice reinforced their rank and good breeding. The ability to express themselves eloquently through the pen was more of an achievement and testament than through speaking. Women were not encouraged to partake in this manly pastime!
Then the 18th century came along and fashionable women of good breeding and families began to write letters. There were writing guide books and manuals available so it became easier for them to learn how to compose graceful correspondence.
Mid way through the century a fabulous book was printed, The Ladies Complete Letter-Writer. This book, published in 1763 was the first sign of a new popularity for women letter-writers. What was unique about the book was that it was made for a specific audience, the ladies!
By the end of the century women had proved so skilled and eloquent in writing it was thought that they were better at it than the men! This gave the art of letter writing new meaning. Rather than separating the men from the women it separated the classes. (The higher classes set the standards and practiced refined letter writing, while the lower classes were assumed vulgar and lacking of any eloquence via correspondence!)