Some historic correspondence between women include letters between Julia Child and Avis DeVoto and Eleonor Roosevelt and Lorena Hickok. The former corresponded beginning in the 1950s, while the latter began their friendship and correspondence in the 1920s.
Due to the importance of letter writing as a form of communication, many women wrote letters to a variety of people. Some of the more well-known historic letters written by a woman are those of Abigail Adams to her husband, president John Adams.
The Internet features many databases featuring letters written by women. For example, the Epistolae section of Columbia University's website features letters written between women and other correspondents during the Middle Ages.