A mailbox is where news from outside arrives, read as expectation and openness to contact. An empty one stands for waiting to hear from someone, a full one for things left unanswered.
When this appeared in your dream, what were you feeling?
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.