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 - Postman
A postman means that you will receive news; if he wears white then expect happy news, but if he’s dirty and unshaven then the news will be bad.