A rouge employee willing to endanger their career/freedom (jail time) with correct access/motivation could illegally access email.
It has happened
I'll use Google as the example here because it actually happened in 2010 where there were 2 alleged employee that breached policy and read emails/chat:
Google acknowledged Wednesday that two employees have been terminated after being caught in separate incidents allegedly spying on user e-mails and chats.
So the same thing could happen at Yahoo, or Microsoft, or any other type of provider that has some kind of administrator access to the infrastructure.
Never send anything sensitive via email, including (but not limited to) Credit Card info, passwords, your most private deep dark thoughts.
You can encrypt your email if paranoid, but this is not a trivial task:
Having said all that, it's actually not a big worry
Unless an employee at the organisation hosting your email is likely to directly target you and your email, don't worry about it. If you believe you are a target, then you may want to consider reporting their threats of undertaking illegal activities to their employer.
Also there's usually only a few employees who actually have such authority/access to do such a task, from this article:
A small number of GMail related engineers have access to the servers as a matter of necessity to do their jobs; a very small number of people actually access the contents as a matter of necessity to do their jobs, and even then, almost always only the associated metadata.