An apparatus for determining the thickness of passing mail items M conveyed on a conveying deck (12) of a mail processing machine (10) comprising a deflecting element (18) which is positioned to successively contact without active compression each conveyed mail item, measures a parameter that indicates a change in a resistivity of the deflecting element, and generates a signal indicative of the measured parameter; and a processing unit (24) which receives the signal from the deflecting element and determines a thickness value for each conveyed mail item based on the measured parameter.