In a method of processing mailpieces, signatures are derived from images of the mailpieces and serve as identifiers for the mailpieces in the sorting process. The signatures are grouped together 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. For each graphical class in question, activity of the mean signature value is analyzed for detecting significant activity of an information block in the digital images. The results of this activity detection are used for searching for a match, in a second sorting pass, between a current signature and signatures recorded in the first sorting pass.