Abstract: A mail slot box includes a bottom, opposite sides, a semi-circular back, and an open top. A mounting flange extends along the front edges of the bottom and sides for mounting behind a mail slot and collecting mail delivered therethrough. The semi-circular back causes the mail to rotate and stack in an orderly fashion without binding or bunching up. When mounted behind a mail slot on a garage door, the semi-circular back provides a concave surface for holding the mail and preventing it from falling out when the garage door is lifted to a horizontal open position.