A mail system having high security is realized by mounting TCP2 for mail communication between client apparatuses. The present invention relates to a mail communication system which is connected to a network and exchanges mails between client apparatuses provided with the existing mailers, and each client apparatus is mounted with a TCP2 driver. A TCP2 driver 34 includes a TCP2 core 36 and a mail system core 37 and an e-mail received via the network is processed in this TCP2 driver 34 and thereafter, is supplied to an existing mailer 31 of the client apparatus. In the mail system core 37 of the TCP2 driver 34, control of mail encryption and decryption, deletion of an unnecessary mail and the like is carried out.