PURPOSE: To prevent an undesired in-LAN subaddress from being stored in a mail server by sending an in-LAN subaddress for its own mail server to an opposite party when a message is sent between mail servers.

CONSTITUTION: The system is provided with a LANA 2-1 to which a mail server A1-1 and clients 3-1, 2-3 are connected, a LANB 2-2 to which a mail server B1-2 and clients 3-3, 3-4 are connected, and a LANC 2-3 to which a mail server C1-3 and clients 3-5, 3-6 are connected. Then a mail server stores only an in-LAN subaddress table for the mail server and an in-LAN subaddress table for other mail server receiving or sending a message and does not store the in-LAN subaddress table not sending the message and then it is prevented that an undesired in-LAN subaddress table is stored in a storage device of the mail server.