A transmitting-end terminal device transmits an electronic mail including large-size data such as video data to a transmitting-end cache via a transmitting-end router. The transmitting-end cache temporarily stores the received data. A POB server authenticates a user of the transmitting-end terminal device and determines whether a receiving-end terminal device can receive the electronic mail. If the authentication of the user is successful and if it is determined that receiving-end terminal device can receive the electronic mail, the transmitting-end cache transmits the electronic mail directly to the receiving-end cache without passing it through the POB server. The receiving-end cache stores the received electronic mail and transmits a message, indicating that the electronic mail has arrived, to the receiving-end terminal device. In response to a request issued by the receiving-end terminal device, the receiving-end cache supplies the electronic mail stored therein to the receiving-end terminal device.