Receiving mail in a mailbox can represent the idea of receiving communication from someone, whether you want it or not (since you have no say over what gets put into your mailbox).
Mailing a letter can represent something you’d like to say to someone else (written, spoken, etc.), a message you’ve sent in real life, or one you’d like to send.
If the mailbox is full, then it indicates that you have not accepted these messages. [2]
2. If box contains mail and the door or flap is open, it indicates good news.
3. If door is closed, bad news. [3]
2. Invitation;
3. A dream or Vision coming;
4. Written communication; 1 John 5:11; Matt. 13:19. [4]