That's one of my problems with yahoo. The site uses https during login but then drops it. If you manually switch back over you get an error because the "reading mail" server doesn't have ssl enabled.

With Google, it uses https during login then flips over to http when your sent on to the reading mail screens. If you manually change back to https, you get the closed tunnel.

It would be better if they just defaulted to https since the login confirms that the browser can manage. Being able to change back is better than not available at all though.