To provide a means by which a situation leaking the personal information of a user is prevented and the user can negotiate directly with another person.

A PES 1 functioning as a mail server sets the mail address of an agent other than the personal mail address of the user in response to a request from the user. The PES server 1 receives electronic mail in which an address is the mail address of the agent, then converts the mail address of the agent in the received electronic mail into the personal mail address of the user, and transmits the electronic mail to a user terminal 3A, ..., in accordance with the mail check by the user. When receiving the electronic mail transmitted from the user terminal 3A, ..., the personal mail address of the user is converted into the mail address of the agent. The electronic mail is transmitted to a provider terminal 4A, ..., being a destination, making a sender as the agent.