An attempt is made to make an incoming notification for a reply to sending mail, an incoming notification related to the sending mail, and an incoming notification from an Internet service provider to a user. Accordingly, when the mail is sent from a communication terminal, if an incoming notification condition related to the mail is added and the mail is sent to a server unit, the server unit detects and registers the information as the incoming notification condition and sends the mail to the Internet. Further, when the mail that matches the incoming notification condition was received, the incoming notification is made to the communication terminal and the condition is changed. Consequently, the incoming notification condition can dynamically be set and changed at mail sending and receiving. That is, the incoming notification condition related to the sending mail can be registered in the server unit at mail sending. When the mail that matches the incoming notification condition was received, the mail is posted to the user as the incoming notification and the incoming notification condition can be changed.