Leigh receives a postcard that is addressed to her at "433 E. 31st St" in New York City. That address no longer exists - that entire neighborhood was cleared in 1945-6 to make room for the New York University Medical Center, which occupies that spot to this day.
Near the end of the movie, Leigh receives a postcard from Jason. Although there is a faint hint of a cancellation stamp, there is no POSTNET bar code at the bottom right of the card. Obviously, this postcard never actually went through the mail.