When an alias mail having an alias address X as a destination is received from an originator terminal, an alias mail relay server restores a recipient address R and an alias address generation argument C, generates a reply destination address Y including the generation argument C restores and an originator address S, and replaces the destination and a transmission source with the recipient address R and Y to transfer the alias mail to a recipient terminal. On the other hand, when a reply mail is received from the recipient terminal, a remailer restored the originator address S and the generation argument C from Y, regenerates X from the generation argument C restored and the recipient address R, and replaces a destination and a transmission source with the originator address S and X to transfer the reply mail to the originator terminal.