Method and apparatus for printing postal code markings

The method of the present invention is intended to facilitate routing of correspondence to a destination address by automatic mail sorting apparatus comprising the steps: (a) marking a letter with an easily machine readable first code (36) corresponding to the human readable postal code of the destination address; (b) stuffing the letter into a window envelope such that the first code (36) is positioned within the window (29); (c) machine reading the first code (36) within the window (29); and (d) printing a machine readable second code (49) corresponding to the first code (36) and the human readable postal code of the destination address in the location and format utilized by a particular automatic mail sorting apparatus such that the envelope is suitable for processing by the apparatus. The apparatus of the present invention includes a conveyor track for passing window envelopes through a read station and a print station. At the read station, an optical sensor reads the first code, in the form of printed marks signifying the postal code, through the window, the information being passed to a computer. At the print station, the second code, in the form of fluorescent or other marks are placed on the envelope, as instructed by the computer.