Select a message to read.
Your device will create an encryption key. Mail is sealed to it so the server stores only unreadable ciphertext. Choose a backup password — you'll need it to add another device. It is never sent anywhere.
Write the password down somewhere safe. If you lose it and all your devices, your stored mail cannot be recovered — that is what "only you can read it" means.
Enter your backup password to load your key on this device.