Emails are received and the received emails are classified and accumulated based on relay path information contained in the header of each email. For each of the classified groups, a judgment is made according to a judgment rule including at least (1) the number of received emails per unit time is a first threshold or more and (2) the number of emails having an invalid destination address is a second threshold or more. For emails included in a classified group satisfying at least a prescribed number of items of the judgment rule, a mail process including (a) blockage of delivery to the user and (b) transmitting a pseudo error message replicating an undelivery mail to the source is carried out.