When you put mail in the box, unless it's a REALLY small bit of mail it'll land so it obscures at least one of the proximity sensors. This then sets the 'got mail' statue to 'on' in Home Assistant. From there, I have HA set up to send me notifications to go and check the mail.
Before you say so, yes this was a lot of work for something so trivial, but it was fun. Plus I actually get so little physical mail that I can forget to check the mailbox for weeks at a time. Which would be very bad if I got some actually important mail. And actually, that exact thing happened just days after I finished installing the thing. So it has already potentially saved me from a fine.