A mailing list server sends mail encrypted by an encryption method compatible with the mail user agent device likely to receive the mail. A mailing list server 20 receives a signed mail from a member and registers and manages the SMIMECapabilities of that mail in a members' encryption function map 212. A mail-receiving part 201 verifies the signature on a mail received from a member using the member's certificate and decrypts the mail. A mail-distributing part 202 signs and encrypts mail for distribution and distributes it to members' mail addresses. A symmetric encryption key to be used for the encryption is determined for each member with reference to the members' encryption function map 212.