A mail receiving section receives S/MIME signature mail from a host of a member. A member checking section checks the electronic signature of the signature mail for validity and determines whether or not the member belongs to a group. If the member checking section checks that the mail is from the valid member, a mail storing section requests an archive service section to archive the mail. A signature creating section uses a secret key unique to the group to create an electronic signature of the group. A mail transmitting section transmits the mail with the electronic signature of the group to the receiver.