A mailing list server and a mail re-sending method thereof with which, when a mail recipient has been unable to decrypt a mail, it is possible to re-send the mail compatibly encrypted in a form such that the recipient can decrypt it without fail. A mail re-send request from a member is made by HTTP, received by a web server 20, and processed by an application server 21. For example, re-send request information is sent to the web server 20 by a mailing list member, using an input form of a web page 303 from the web server 20. The application server 21 determines an encryption method on the basis of a user encryption method specification included in the mail re-send request. A sending part 101 encrypts the mail by the determined method and sends it to the member's address.