[jjav]

"Inbox" used to be literally the box/tray on your desk where people would leave work for you in written letters. That's where email got the term.

Amusingly in my first job in the 90s they still assigned each desk a labeled inbox and an outbox (two trays on the desk), but by then it was all email already so nobody ever used those on my desk. But they sat there for a few years as a memento to the 80s!