A method for sorting mail to a case having a plurality of slots is described, wherein each slot corresponds to a destination. The method includes steps of loading a mail piece to be sorted into a delivery robot, determining for the mail piece a destination slot the mail piece is to be delivered to, moving the delivery robot along a rail disposed at the front of the case near the slots into proximity with an open end of the destination slot, inserting the mail piece from the delivery robot into the associated slot, and returning the delivery robot to a loading station whereby the cycle may be repeated. Such a method, when using a large number of robots moving along a common rail system, can sort a large volume of mail in carrier delivery order.