Is it illegal to read someone's email without their knowledge? It depends.
Have you worried that somebody is reading your mail and still there is no way to track down?
But there is an easy solution to solve the problem.
Email providers can add a feature which gives us information when the user last logged into the email account and 'hacker' of your email cannot tamper this info.
If you find that last login time changed, go and change the password right away and protect your personal emails.
Is gmail, yahoo and other email providers listening ??