When looking at search results, it would be extremely useful to be able to find where an email is, so that the rest of the conversation could be followed. Two ways I see to do this:
1. Have a "Folder" column, which shows where each email is stored. This is the minimum.
2. Add some kind of "Show location of email" option which opens the relevant folder, and highlights the email in it.
If displaying the logical folder is too hard, please at least show the physical folder (i.e. directory path).