Accessing Yahoo email via their webmail portal has always been a somewhat insecure affair. This was because while logging on to Yahoo was always SSL encrypted there was no encryption for actually accessing your emails in the browser. This left sessions vulnerable to potential snooping attacks and perhaps worse, especially if the user was accessing it from a public Wi-Fi hotspot. However, users on Google’s or Microsoft’s email services had no such problems, because those sessions have been SSL protected for a long time.
However, full SSL protection has now finally been implemented by Yahoo, so to turn it on, simply tick the option “Turn on SSL” in the General settings. Instructions for how to go about this are provided by Yahoo, here.