I use Outlook 2007 without any add-in. My projects have their own folders where I put any related email after dragging it to the To-Do Bar. The To-Do Bar is set up to view by category, which are my contexts. This makes using the To-Do Bar the best thing in Outlook for me: it assigns the category and a due date. Right now, I don't have due dates set, I do that in my Weekly Review. I can glance down the To Do Bar, open the context I'm in, and knock out the tasks. Plus, I can change the task to something more action oriented ("add to next version" instead of "application request 23492")
When that project is complete, I move it to an archive pst file.
How do you set it up to automatically add the category? I tried to drag a message to the To-Do Bar but nothing happened. I do drag messages over the Tasks shortcut in the left side and manually add due dates and categories for contexts. I also have my To-Do bar set up to view tasks by category which I use for GTD contexts.