Have you turned Yahoo Mail's SSL on?

I didn't notice that this past January, Yahoo Mail had enabled SSL (HTTPS).  Of course, they didn't enable it across the board, rather, they hid it as an option.  Under Mail Options, in the Advanced Settings, there's a little check box to Turn on SSL.  When you check the box, you'll get a pop up window with a spelling error; hard to believe that no one has caught and fixed it.

Don't forget to hit the Save button.