$A$ has a message that he wants to send to $B$, without a third party being able to learn the contents of the original message.
$A$ encrypts the message and sends it to a server. The server decrypts the message somehow, and sends it to $B$. This way, no one would know what was the original sender, because the server will send it to the destination.
But if the server encrypts the message again, how will $B$ be able to decrypt the message? If $B$ knows how to decrypt the message, then any third side would also know.
What is the solution?