It depends. If they have physical access to your computer it is relatively easy to access a computer and any email that is stored on it. Other than that it would take a some form of remote desktop misconfiguration or a malware infection to get access. If they are able to find out or guess your password they won't need access to your computer to get access to at least some of your email. I recommend enabling 2-step verification for any email services you have that support it.