MAIL SERVER

PROBLEM TO BE SOLVED: To provide a mail server that can transmit an electronic mail with a digital signature to a destination, without increasing traffic of a data transmission channel and has a security effect of the digital signature, even when a mail transmitter terminal has no configuration for the digital signature. SOLUTION: A gateway server 22 contained in a mobile packet communication network 20 which is a closed network receives electronic mails from mobile stations 10A, 10B, etc., and applies a digital signature to the received electronic mails. Then the gateway server 22 transmits the electronic mails with the digital signature applied to them to a personal computer 40A through the Internet 30. The personal computer 40A analyzes the received electronic mails and the digital signature to detect the existence of forgery.