Thanks for your kind reply. When you say "legal means" you mean "sue" them ? I could go this far but if there is something in between it would be better but I don't know if it exists...
In good faith, I'm pretty sure I remember the security answer I have provided, I had a screenshot of the yahoo mail app when I was login and I got some logs of the application which state that I was indeed log in years ago... I even jailbreak the related phone to take a look of what it's called "keychain" but the yahoo app used token authentication or I never "saved my password" :/
Are you aware of any special contact (mail, phone, whatever) for something like this ?
Many thanks anyway.