We have a subscription service on our site that requires our customers to login. We provide monthly update notices to our customers via simple text emails, with a link to the login page. Once customers are logged in, they can view their updated content. We've had this service for about two years, and I've never seen this issue before.
We recently began getting complaints from customers who logged in successfully, but then were immediately logged out with any subsequent click on any internal link on our site. We couldn't figure out what was going on, until we asked one of the customers to send us URLs and screenshots from his browser.
It turns out that all the customer complaints were from people with MSN or Hotmail email addresses. It turns out that MSN/Hotmail changes the URLs of any links in our email, and redirects the links through a little top frameset that preserves their branding, but totally screws up the user experience on my site. My site uses the browser's URL to set context for subsequent links. Whenever a customer clicks on a link from the initial login landing page (post login), the context is destroyed by the frameset, the site thinks the session has ended, and the customer gets directed to the login page. This results in an endless loop for the customer, and a crappy experience.
It's definitely a warning to anyone who depends on email to deliver visitors back to their site.
I searched WW and couldn't find anything about this topic. I don't know if MSN/Hotmail have always been doing this, because I don't use any of their services. But they're doing it now, and it's a good reason to use framebuster code, especially if you're an emailer. We're working on putting that into place, and hopefully it will correct the problem.