A private branch exchange (PBX) capable of transmitting and receiving an E-mail over a network is provided. The PBX includes an interface for communicating with the network, an E-mail server for receiving and storing the E-mail; an extension interface for exchanging a communication signal including an audio signal and control signal with an extension telephone, a database for storing a destination address of an E-mail and an extension number corresponding to the address, an examining unit for examining whether or not an mailing address contained in an E-mail received at the E-mail server from the network is registered in the database, and a controller for sending a notice of the reception of the E-mail the extension telephone of the extension number allocated to the mailing address of the E-mail when the examining unit judges that the mailing address of the E-mail is registered in the database.