Self-contained device for attachment to rural mailboxes which automatically signals when the mailbox door has been opened, including a channel support member, a signal flag pivotally connected to the channel support, a trigger element pivotally connected to the signal flag and shaped to cooperate with the channel support in setting the trigger mechanism, and separate biasing means for the signal flag and trigger element which allow the trigger mechanism to be easily set by a simple rotation of the signal flag and to be automatically released by an opening of the door of a mailbox upon which the device is mounted.