Puzzle | Secret Mail Problem

A person ‘A’ wants to send a secret message to his friend ‘B’ through a mail, but C (A’s friend), whom A does not trust has access to all A’s mails. So A puts his message in a box with a lock but A is not allowed to send a key. How can A send his message through securely?

Solution:
A send the box to B.
B can not open it.So ,he can put another lock on the box.
B send this box with the 2 locks send back to A. A unlock his lock and send it back to B again.

So, there is just B’s lock on the box. B can open it.