1. A postal sorting method in which mailpieces are subjected to at least two sorting passes in a machine so as to be sequenced in sorting outlets of the machine, and user processing information is available in the machine during a first sorting pass in relation to at least a certain mailpiece, said postal sorting method comprising, in a memory of a data processing unit, associating said user processing information with a graphical signature of said certain mailpiece that is representative of an image of said certain mailpiece during the first sorting pass, and recovering, during the second machine sorting pass of said certain mailpiece, said user processing information that is associated with said certain mailpiece on the basis of matching of graphical signatures between said first and second sorting passes, using a graphical resemblance criterion; wherein, while a set of mailpieces are being recirculated into the machine for a second sorting pass, graphical signatures associated respectively with user processing information are filtered for the purpose of dynamically creating a dictionary of graphical classes, and wherein matching is detected between a graphical class and a graphical signature of a mailpiece as computed during the second sorting pass for the purpose of recovering the user processing information associated with said mailpiece.