Best answer as chosen by user foreversignedin: What is "an old session"?
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.