1. A method of processing mailpieces comprising, during a first mailpiece sorting pass, forming a digital image of the surface of each mailpiece that bears information blocks, deriving a digital signature from each image associated with a mailpiece, which signature is a logical identifier for the mailpiece, and recording the signature in a memory in correspondence with sorting information, and, during a second mailpiece sorting pass, forming a digital image of said surface of each mailpiece again in order to derive a current digital signature associated with the mailpiece, and in searching through the signatures recorded in the first sorting pass for a match with said current signature in order to retrieve the sorting information by association, wherein said method further comprises the following steps: grouping together the signatures into signature categories or graphical classes on the basis of a certain criterion of graphical similitude so as to compute a mean signature value for each graphical class; analyzing, for each graphical class in question, activity of the mean signature value for the purpose of detecting significant activity of an information block in the digital images; and using the results of this activity detection for the purpose of searching for a match.