E-mail is basically the same as "Postcard".
The Internet is a set on two or more networks. Unfortunately, the person who steals a glance at the mail that flows in each network and falsifies exists.
It is a reason of not transmitting secret and high information by E-mail.
This application, "Josuijo-Dori" was made for to encrypt the attached file of mail.

People often mistake send E-mail to non-target Addressee.
Contents of the attached file are never seen even if it transmits to a wrong other party if the attached file is encrypted by Josuijo-Dori.

Because it doesn't have a corresponding private key though a wrong other party of the transmission also is using Josuijo-DOri, it is not possible to decrypt it.