When you recieve a private message that was deleted by it's author and you want to view it, you get a delete screen. After deleting it, you're redirected to:

http://www.phpbb.com/community/ucp.php?i=pm&mode=view&action=view_message&p=683631

Which then gives you the error:

Information

You are not able to read this message because it was removed by the author.

Despite that you're redirected to that location, you should still have a way to return to the inbox. I suggest a link back.