I'm 99.99 percent sure I wasn't hacked
What i suspect is you have logged in to your email id after a longer duration of time, could be after a year?
Yahoo does deletes the email, so that is a YES, but they don't delete it if you login and check your account at-least once in 1 year or so.
If your emails are deleted then the case might be that you have not logged in to your yahoo email account for a longer duration of time.
The same is the situation with Rediff, if you don't login to your Rediff mail once in a year you loose your emails and there is a restoration fee that you need to pay. This happened with me once then i just stopped using Rediff.
Gmail is the best and would always be the best.
I did not log in for over a year on that account and if what you are saying is true this REALLY sucks and have lost a lot of important emails.