Abstract: A paperboard mailing envelope of the type having an interlocking tongue and slot formed one on the closure flap and one on the back panel. The tongue and slot are adapted to interlock to prevent undetected withdrawal of the locking tongue from the slot. A detachable tear strip margin is provided on each of the front and back panels. The margins connect adjacent side edges of the front and back panels. The detachable tear strip margins are severable from their associated panels along weakened tear lines to provide an unloading opening through which the contents of the envelope may be removed.