Sending and receiving end-to-end encrypted emails
The email is encrypted on Alice' client, stored encrypted on the server and can only be decrypted by Alice or Bob.
Sending non-confidential emails
The email is sent in via SMTP to the recipient. Still, the sent email is encrypted for Alice on the server and then stored.
Receiving non-confidential emails
When the SMTP email is received by the Tutanota server, it is encrypted for Alice and then stored on the server.