I am looking for a web-UI email service where the entire session is SSL (encrypted). Gmail and Yahoo Mail are both web applications but SSL is only used for the login page, the rest of the session (including emails) are in the clear. I am willing to pay for such a service. It would of course have to have all the latest email features (search, folders, contacts, etc) and be reliable.
You can force Gmail to use SSL the entire time. Scroll down to "Browser connection" on the first page of Settings, and check the "Always use HTTPS" option. And Done :D
Edit 4/29/2014: This is now enabled by the default but can be turned off. Also, Google Apps admins can allow or disallow the option to turn it off in Google's cpanel.