Patent ID: 8341418

Claim:
An electronic mail transmission/reception system comprising a first server device capable of transmitting/receiving via a network electronic mail composed of a mail body and an attachment added to the mail body, wherein, the first server device includes: first conversion means for hashing a destination restricted attachment intended for a predetermined destination for which a destination restriction identifier indicating its destination is set by a one-way hash function, and converting the destination restricted attachment into a first hash output value; first storage means for storing the first hash output value for the destination restricted attachment converted via the first conversion means; second conversion means for hashing a received attachment other than the destination restricted attachment by the one-way hash function, and converting the attachment into a second hash output value; second storage means for storing the second hash output value for the attachment converted via the second conversion means; and destination restriction means for comparing the first hash output value with the second hash output value, and when these hash output values are equal, determining that the attachment corresponding to the second hash output value is identical to a destination restricted attachment corresponding to the first hash output value and transmitting the attachment corresponding to the second hash output value only to a destination indicated by the destination restriction identifier based on the destination restriction identifier set for the destination restricted attachment, wherein the first server compares the first hash output value with the second hash output value, and when these hash output values are different, determines that the attachment corresponding to the second hash output value is different from the destination restricted attachment corresponding to the first hash output value and transmits the attachment corresponding to the second hash output value to the destination indicated by the attachment.