Years ago, when online check-in was still in its infancy, I got something emailed to me from Southwest... A boarding pass perhaps? Maybe we weren't able to print them straight from the screen. Anyway, it went into my spam folder and when I noticed it there it was just a jumble of HTML code. Fortunately I was able to find the confirmation number amongst all the other stuff and printed the page and took it to the airport and had an agent print me a new boarding pass. Annoying, but it worked.