Tutanota is an email provider with a focus on security and privacy. Besides email, they also offer calendar and contact management. The biggest feature of tutanota is that data like emails, calendar and contacts are end-to-end encrypted.
This means that not even the provider itself can read the data because they are decrypted only at the client. This also means that only the clients of tutanota can be used to retrieve mails or edit the calendar. tutanota offers clients for the browser, iOS, Android, Windows, Mac and Linux.