To provide a function that a mail server can automatically inform a sender about a new mail address of a recipient electronic mail user even when the sender transmits an electronic mail to the old mail address of the recipient soon after the recipient electronic mail user changes its mail address.

In the case that a mail user X transmits an electronic mail to a mail user A (to old mail address) after the mail user A changes a folder destination of electronic mails from a server 2 into a server 3 and that the mail server 2 receives the electronic mail, the mail server 2 transfers the received electronic mail to a new mail address of the transfer destination read from a database 25, reads the sender mail address from the header information of the received electronic mail, generates an electronic mail to inform the mail user X of the revision of the transmission destination mail address and returns the generated electronic mail to the sender mail address.