Abstract: A mail processing system includes mail processing devices such as postage meters, a sorting machine and a host computer. Each mail processing device provides to each outgoing mail not only its mailing charge but also an indicator which identifies the device itself, storing such provided data in its memory device. The sorting machine reads from each received mail the data provided to it and stores such data in its memory device according to the device-identifying indicator. The computer serves to retrieve data from these memory devices to make comparisons. If the discrepancy is greater than a specified value, a warning signal is outputted as a fraud has been committed.