Have you ever deleted the wrong document from your inbox and you knew it the moment it happened? Then you have to go to the trash folder, find the message you deleted, click restore, then return to the inbox and act on it. I would like the undo feature to include undoing document deletions.
Just like there is an Edit - Undo menu option available for text editing, the undo feature should also be available for document deletions.
Have you ever dragged a message to a folder and the folder list expanded or scrolled just as you dropped the message? Now you don't know which folder it was moved to. So you have to go to all documents, find the message and identify where it went. Undo should also be available for folder moves.
There are probably numerous other actions that the undo option should act on, but these come to my mind right now.