Abstract: An automatic signaling device to be used in conjunction with a standard rural mailbox to indicate that the door has been opened when a mail carrier puts mail in the mailbox. The primary components of the device comprise a signal flag, including a helical spring and a signal plate for attachment to the top end of the spring, a mounting bracket for mounting the helical spring to the door of the mailbox, and a retaining bracket attached to the mailbox top. When in a set, or cocked, position, the spring is bent and the retaining bracket retains the signal plate in a horizontal position over the top of the mailbox. Upon opening the mailbox door to deposit mail, the spring unbends thus permitting the signal plate to be held in a vertical position above the mailbox top when the door is re-shut.