mail slot

From mail +‎ slot.

mail slot (plural mail slots)

1. A slot in a wall or door through which mail is delivered.
   • 1989 April 15, Michael Ambrosino, “Acting Up In The Northwest”, in Gay Community News, page 16:

     Stuffing ACT UP leaflets into the office door's mailslot.

• letterbox

• maillots, misallot