| 摘要 |
PROBLEM TO BE SOLVED: To enable an electronic mail transmitter to designate the date of opening an electronic mail. SOLUTION: This method includes to transmit by adding the opening date to the electronic mail and to prohibit opening the mail when the current date are later than that of opening. Such prohibition is performed by encoding the electronic mail and decoding it only when the current date is earlier than that of opening. Furthermore, the opening date is changed when the current date has been changed to that of others, based on the current date and that of others, and the relative time of the current and opening date before change is equal to the relative time of the current and opening date after change. The change of this opening date makes it impossible to open the electronic mail through the change of the current date by the receiver.
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