A method for operating a machine that processes printing substrates in sheet form, in particular, a printing press, including a delivery device (1) which contains a drive (2) for conveying the substrate sheets and at least one opening (8) through which the substrate sheets are conveyed from the inside (4) of the delivery device (1) to an externally accessible region (6), and further including a control device which controls at least one closing device (7) for opening or closing the opening (8) and which is operatively connected to the drive (2) of the delivery device (1). Also provided is a device for carrying out the method. The present invention has the feature that the opening (8) is closed or is closed by the control device when no substrate sheet is conveyed through the opening (8).