A letter is read as news from a part of yourself. Dream books take it as something that wants to be said, or heard. A letter you cannot open or read points to a message whose time has not come yet, while writing one is read as the wish to express what is hard to say aloud. A letter from someone far away or long gone is traditionally taken as memory and longing. An unsent letter is read as words held back. Receiving many letters suggests a busy, outward-turned time.
Does the traditional reading fit your dream, or did it mean something else to you?
My Cosmic Code has a private dream journal. Write a dream down and the app picks out the symbols in it, shows what dream books say about them and, over time, which words recur across your entries. The journal is stored on your device only.