To see a mail box in your dream, represents important information that you are about to receive. Pay particular attention to your dream and what message it is trying to convey to you from your unconscious. If the mailbox is full, then it indicates that you have not digested or accepted these messages.
Receiving mail in a mailbox can represent the idea of receiving communication from someone, whether you want it or not (you have no say over what gets put into your mailbox). Mailing an outgoing letter can represent you communicating with someone else.