A physical receptacle into which postal carriers deliver incoming mail, to be picked up by the recipient. Also known as a letterbox*, letter plate, letter hole, mail slot, etc. (Wikipedia. Some mailboxes feature an arm that can be flipped up to indicate that there is some outgoing mail within for the postal carrier to retrieve when they come by delivering mail.
For tagging purposes, distinct from postbox (outgoing mail), which is for outgoing mail.
*However, letterbox is used here to refer to the image composition technique.