in the exemplary embodiment the object of which the stiffness is to be automatically determined is a flat mail item , e . g . a large letter . a sequence of flat mail items arrives in a sorting center . these mail items are to be processed by a sorting system . this sorting system automatically reads the delivery address of each mail item and extracts the mail item depending on the recognized delivery address into one of many output compartments . in such cases each mail item is transported through the sorting system . when this is done the direction of transport is changed more frequently . each flat mail item extends in an object plane . it is possible that a mail item is not stiff enough and that only a too low force is required in order to deflect the mail item at right angles to the object plane . if such a mail item ( also referred to as a “ flimsy mail item ”) were to be processed together with the other mail items by the sorting system , there is the danger of the mail item that is not stiff enough being damaged or even destroyed on its journey through the sorting system . in addition the transport of this mail item can cause a blockage , especially before an output compartment , because the mail item becomes creased for example . it would not be sufficient to merely measure the thickness of each mail item . mail items of equal thickness can have significantly different degrees of stiffness for example , since their respective content differs in its stiffness . it is thus necessary , for each mail item of the sequence , to test whether the mail item possesses a stiffness sufficient for automatic processing or not . a mail item that does not possess sufficient stiffness is extracted by the shortest paths into a special output compartment and preferably manually processed . the figure shows a schematic overhead view of the test device . two mail items are shown in this example : one mail item b 1 and a subsequent mail item b 2 . mail items b 1 , b 2 are transported in a direction of transport t . in the exemplary embodiment the direction of transport t lies in the object plane . the object plane lies at right angles to the plane of the drawing of the figure . a force exertion device 1 in the form of a fan with a power supply 2 , a light barrier with a light source 3 and a receiver 4 , a guide plate 5 for stabilizing the mail items , a further guide plate 6 for aligning the mail items , a contact sensor in the guide plate 5 , a first endless conveyor belt ef 1 which is routed around one driven pulley 8 and two non - driven pulleys 9 , 10 , and a second endless conveyor belt ef 2 , which is routed around one driven pulley 1 and two non - driven pulleys 12 , 13 . the mail item b 1 is held and transported by this holder device . the mail item b 2 currently being examined is held by two endless conveyor belts that are the same as the two endless conveyor belts ef 1 , ef 2 . these have been excluded from the figure for reasons of clarity . the two endless conveyor belts not shown that move a mail item past fan 1 and in the situation of the figure are currently holding the mail item b 2 , function together as a holder device as defined in the claims . the endless conveyor belts ef 1 , ef 2 hold a mail item flat in a strip that extends in the direction of transport t over the length of the mail item . the area of the mail item lying above this strip is movable at right angles to the object plane and thus also at right angles to the direction of transport t and functions as a movement area of the flat object . the fan 1 exerts a force f on the held object . in the embodiment shown in the figure the force f acts at right angles on the object plane and thus also at right angles to the direction of transport t . in the situation shown in the figure it acts on object b 2 . in another embodiment the force f acts at an angle to the object plane . in both embodiments the force f exerted has a force component at right angles to the object plane . preferably the fan 1 blows air from the environment against the mail item b 2 to be examined . in dusty or fire - hazard environments it is however typically advantageous for the fan to instead pump out a fluid ( gas or liquid ) from a storage container and blow it against the mail item b 2 to be examined . the effect of the force f produced by the fan is to deflect the movement area of the object by a distance at right angles to the object plane . in the exemplary embodiment the force causes a deflection at right angles to the direction of transport t , i . e . in the figure a deflection to the right . the figure shows that — viewed in the direction of transport t — the rear upper corner of mail item b 2 is strongly bent . in the exemplary embodiment the mail item b 1 has already passed the test device . it has been established that the mail item b 1 has a sufficiently great stiffness . the fan 1 has only been able to deflect the mail item b 1 by a slight distance ( much less than dist ). the light barrier has not registered a deflection greater than dist . the test device is now examining mail item b 2 . the fan 1 — viewed in the direction of transport t — causes the rear upper corner of mail item b 2 to be bent over and thereby deflected . the transmitter 3 of the light barrier emits a light beam ls or even an infrared beam , and does this in parallel to the object plane and in the exemplary embodiment thus in the direction of transport t . the receiver 4 measures whether the light beam ls from the transmitter 3 arrives at full strength or whether the deflected mail item b 2 interrupts or at least weakens the light beam . so that the deflected mail item b 2 interrupts the light beam ls , the deflection must be the same or greater than a predetermined limit dist . this distance dist is simultaneously the distance between the object plane of the object , while the latter is being moved past the force application device 1 , and the light beam ls . if a deflected mail item interrupts the light beam ls , the deflection is the same or greater than dist . in this case a decision is automatically taken that the stiffness of the mail item is not large enough . in the example shown in the figure mail item b 1 has a sufficiently great stiffness whereas mail item b 2 is not stiff enough since the deflected mail item b 2 interrupts the light beam ls . in the exemplary embodiment the deflection of mail item is at its greatest at the topmost edge . preferably the light barrier 3 , 4 is designed so that the light beam ls has a sufficiently large coverage in the vertical direction to detect mail items of different height . preferably a suitable value for dist is determined empirically . to this end mail items which have a sufficient stiffness and also mail items which are not stiff enough are transported for trial purposes by the device of the figure and in this case are routed past the fan 1 . the fan 1 creates the same force each time , which acts on the mail item . the size of the deflection is measured for sufficiently stiff and for insufficiently stiff mail items . the value dist is set so that the light beam ls will only be interrupted by insufficiently stiff mail items . if the available space is not sufficient to achieve a distance dist between light beam ls and object plane , the force f that the fan 1 exerts on the mail item will preferably be reduced . conversely it is possible to increase the force if instead a sufficient distinction between stiff and insufficiently stiff mail items is to be achieved . in the exemplary embodiment the force exertion device 1 is embodied as a stationary device , and an object b 1 , b 2 to be examined is moved past the device 1 . an alternate embodiment is however also possible , in which conversely the force exertion device 1 is moved past the object b 1 , b 2 to be examined . for example the fan 1 is mounted on a rail which runs in parallel to the object plane . the fan 1 is guided past the held mail item b 1 , b 2 . in the exemplary embodiment the relative displacement between the object to be examined and the force exertion object 1 occurs in parallel to the object plane . it is also possible for the relative displacement to occur in another direction , for example by the mail item being moved at an angle past the fan 1 . in the exemplary embodiment the device further includes a guide device in the form of a guide plate 5 . this guide plate 5 is embodied to limit a deflection of the movement area at right angles to the object plane . a deflection which is too great can actually damage a mail item . a contact sensor 7 is built into the guide plate 5 in the exemplary embodiment . this contact sensor 7 measures whether a mail item touches the guide plate 5 or not . if a mail item touches the guide plate 5 the fan 1 has produced a deflection which is much greater than dist . in this case the stiffness of the mail item is particularly small . the mail item b 2 in the example of the figure will touch the contact sensor 7 . from this it is detected that it has an especially low stiffness . a further guide plate 6 realigns a mail item which has got into a angled position because of the air from the fan 1 . the mail items must be in an almost perpendicular position to enable them to be further processed or extracted without being damaged . a mail item with a stiffness that is too low and which is thus deflected by a distance of dist or more has too low a stiffness . it will be automatically diverted into a special output compartment . in the example shown in the figure mail item b 2 is removed into this special output compartment . it is not processed further by the sorting system but is processed manually .