Ring's Mailbox Sensor monitors your mail

Ring's Mailbox Sensor is a palm-sized plastic sensor, available in black or white, that attaches to the inside of your mailbox door.

Once configured in the Ring app, it's supposed to send real-time alerts every time the mailbox is opened. Adhesive and mounting hardware are included in the box, so you can decide how you want to install it, and you'll need three AAA batteries to power it (not included).