It may or may not be possible to the administrative account that read or access you email account. Here are two scenario take place we will talk about those scenario one by one.
In the first scenario just when you get an email address with your ISP they can provide you more then one email account with your internet connection. It with some ISP you will get full privacy with each and every account so those type of email account is fully private. But some ISP provide one administrative account and the other email account are secondary email account at which the adinistrative account can get full access, so that with the administrative email account someone who log in with this account can read your all emails.
In the second scenario like yahoo mail account at which you can create profiles within the main email account and also you can use those profiles like an another email account, But in this case the main email account have all the accessibility in your profile.
So we can say that it totally depend on the ISP Provider and email provider that wheather they want to give you fully private email address or just a sharing account at which your administrative email account get the access on to your emails.