When you send someone a message, you upload the encrypted message to the server addressed for them. The user checks in with the server asking "any messages for me?" The server then delivers the message and deletes its cache of messages.
The server "stores" the message for that period of time when the user uploaded it and before the receiver confirmed the download. Allegedly, it stores it in the encrypted form as its advertised as end to end so they would not have the key to decrypt it.
The server "stores" the message for that period of time when the user uploaded it and before the receiver confirmed the download. Allegedly, it stores it in the encrypted form as its advertised as end to end so they would not have the key to decrypt it.