At the top of the security section make sure the "Use SSL" checkbox is ticked and don't forget to press "Save." Once that's done, your inbox tab will refresh and you should see a lock icon on the left side of the address bar along with the letters "https." That means your SSL connection is working properly.
If you want to make sure you're using an SSL connection whenever possible, also check out the Electronic Frontier Foundation's HTTPS Everywhere browser plugin for Chrome and Firefox.
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