The protocol is meant to prevent attackers to eavesdrop on the users' electronic communication, especially when using public wifi hotspots, and its use is signalized by the HTTPS expression preceding the URL.
Unfortunately, Yahoo hasn't switched on SSL by default, but has left to users the option to enable it in their accounts. Yahoo! Mail users are advised to do so in their account Options (click on the screenshot to enlarge it):
Of the largest webmail providers, Google (Gmail) and Microsoft (Hotmail) have made the same move in 2010 - Google in the wake of the Aurora attacks, and Microsoft started with HTTPS enabled for login soon after.
It now remains to be seen if AOL will finally follow suit.
By subscribing to our early morning news update, you will receive a daily digest of the latest security news published on Help Net Security.
With over 500 issues so far, reading our newsletter every Monday morning will keep you up-to-date with security risks out there.