Abstract: A mail flow compensating device which is a straight horizontal conveyor belt and is supplied with a stream of overlapping flat mail items. The flow compensating device is made of a stop wall sloping in relation to the belt and presenting a bottom end portion facing the belt. When the height of the stream exceeds a maximum value, the mail items contact and accumulate against the stop wall to form a group of superimposed mail items. The flow compensating device also presents an optical sensor for detecting the limit contour of the group of mail items, generating a full-flow-compensating-device signal, and cutting off supply to the conveyor belt.