Protect premium content - URL's

We have an online newsletter that is "premium" content. A person has to login in order to see this content after they have paid their subscription fee.

My question is... the direct link to this premium content seems to be unprotected. We have a feedburner feed set up. So there are "automatic" emails being generated with a direct link to this premium content. But the link goes directly to the content and does not require the person to login. In essence, anyone with the link could see the content.

If you go to the site directly, it seems to work correctly. If you go to the premium section, you get the "Oops, premium users only" message which requires you to login to view. Which is what we want.

I've tested sending out the link myself in just Gmail and it does the same thing. Viewable by anyone with the link.

Any suggestions on this....

Thanks