Among other things, there is disclosed a system for notifying a user of mailbox activity. A transmitter module and a receiver module are linked together so that when the transmitter module senses the opening of the mailbox, a signal is transmitted to the receiver module. A portion of the receiver module changes color and a sound is emitted to indicate mailbox activity. The receiver module is connected via the internet to a cloud server, which is connected to a user's smart device, so that a notification of mailbox activity is sent to the smart device.