Who wrote the same letter to the three wives in the film "A Letter to Three Wives"?

'A Letter to Three Wives' is a 1949 American romantic drama film which tells the story of a woman who mails a letter to three women. It stars Jeanne Crain, Linda Darnell, Ann Sothern, Paul Douglas in his film debut, Kirk Douglas, Jeffrey Lynn, and Thelma Ritter. An uncredited Celeste Holm provides the voice of Addie Ross, the unseen woman who wrote the titular letter.

The movie was adapted by Vera Caspary and Joseph L. Mankiewicz from 'Letter to Five Wives', a 1946 novel by John Klempner, which had appeared in Cosmopolitan. It was directed by Mankiewicz, who directed 'All About Eve' the following year.

The film won the Academy Awards for Best Director and Best Writing, Screenplay and was nominated for Best Picture.

Just as they are about to take a group of underprivileged children on a riverboat ride and picnic, Deborah Bishop (Jeanne Crain), Rita Phipps (Ann Sothern), and Lora Mae Hollingsway (Linda Darnell) receive a message from Addie Ross informing them that she has run off with one of their husbands. She, however, leaves them in suspense as to which one. All three marriages are shown in flashback to be strained.