I've never heard of that before and certainly not for email accounts where there is no such thing as old sessions. You are either logged in now, or you are not.
So I am as confused as you.
I wonder if this is talking about old web site sessions. Do you log out of Yahoo's email account properly when you finish? I wonder if you need to clear your browser caches.
Or perhaps this... When you log in to Yahoo's email page in your laptop, are you still logged in on your iPhone?
As you can see I am struggling to understand this.
Laptop = Vista IE9 or Safari, Att.net-Yahoo Email. iphone4 = Att.net-Yahoo Mail. When I try SignIn to Att.net-Yahoo Email from laptop directed to an "old session" window (secure site) instead of the usual SignIn page. I can SignIn from this page but changes to unsecure site. Tried turning Att.net-Yahoo Mail off on iphone but same problem. Please help. Thanks