This invention relates to a machine for franking mail items, comprising means for printing postal indicia on a mail item, means for detecting the position of this mail item along a path of travel of this item and control means linked to said detection means and to said printing means for proceeding with printing postal indicia on said mail item at a predetermined place, and means for acquisition and processing of an image of such printed postal indicia so as to determine the conformity thereof with respect to at least one predetermined reference image. These image acquisition and processing means comprise in particular means for determining a lack of registration of said printed postal indicia.