1. A method of processing mailpieces comprising the steps of: forming, during a first mailpiece sorting pass, a digital image of the surface of each mailpiece that bears information blocks, deriving a digital signature from each image associated with a mailpiece, wherein said digital signature is a logical identifier for the mailpiece having an image component representative of physical characteristics of the digital image of the mailpiece and a postal component representative of at least a spatial position of an information block present in the image of the mailpiece, and recording the signature in a memory in correspondence with sorting information; and forming, during a second mailpiece sorting pass, a digital image of a surface of a current mailpiece in order to derive a current digital signature associated with the mailpiece, wherein said method further comprises the following steps: a) comparing said current signature to graphical classes of signatures modeling different categories of mailpiece signatures to detect a candidate graphical class having a certain amount of similarity with said current signature, wherein each graphical class of signatures has an image component corresponding to a consolidated mean value of image components for successive similar current signatures derived in the second sorting pass; b) computing standard deviation values from said current signature image component and said candidate graphical class image component and determining a spatial position of a recipient address information block that has a significant low-frequency activity; c) enhancing reliability of said current signature postal component using said determined spatial position of said recipient address information block that has a significant low-frequency activity; and d) searching through the signatures recorded in the first sorting pass for a match with said enhanced current signature in order to retrieve sorting information by association.