So, in other words, you're not using the Mail application, you're using your web browser to view your email online?
If that's the case, as I stated above, you're going to have to use whatever options your email provider has supplied. We can't know what that happens to be unless you tell us who that provider is, whether it's gmail, yahoo, aol, roadrunner or whomever and then there is someone here that is also using that same email provider.