Here's an idea about how to start with the mailbox sensor: https://www.twilio.com/blog/iot-mailbox-sensor-m2m-to-sms-functions https://www.twilio.com/blog/iot-m

Here's an idea about how to start with the mailbox sensor: https://www.twilio.com/blog/iot-mailbox-sensor-m2m-to-sms-functions https://www.twilio.com/blog/iot-mailbox-sensor-m2m-to-sms-fu.... This one sends an SMS when triggered, but you could certainly add it into Home Assistant too.