Abstract: The apparatus of the invention for discriminating mail in a paper envelope from mail in a plastics envelope makes use of a pneumatic technique. In particular, the apparatus comprises a pneumatic system suitable for deforming the envelope to be inspected by applying suction and for measuring variation in pressure associated with said deformation, whereby the inspected envelope is detected as being a paper envelope or as being a plastics envelope. This pneumatic detector may be associated with a detector operating by means of an optical effect for the purpose of controlling a labelling device in equipment for processing flat articles.