All you have to do is create a mailbox where you are. Any thoughts or secrets you store in it can only be read by the one to whom you entrust the data about the box identifier and the access password, only in the same location. Unless you are a premium user and you have some super powers in your hands.
1
Tell the sender the box identification code and access password. He will have to move to the location where the box was created in order to access it and read the secrets from it.
It's as simple as that!