In the world of dreams the rules of reality do not necessarily apply. Each dream is unique and it's a direct link to the soul and spirit. Psychology provides its own theories on what dreams mean, but so did tradition and the study of dreams for thousands of years before any relevant science had developed.
Dream Meanings - Messenger
If in your dream you are visited at home by a messenger, postman or even a courier then you will have some unexpected happy news or an unexpected visit that will bring you joy.