What are the .mbox files in ~/Library/Mail/Mail Lost+Found/Mailboxes in Mail.app 5.0 (Lion)?
This folder is created by Mail when updating to Lion. I don't think it indicates any problems with a forced shutdown. It's just a place to store the old files after Mail has migrated to its new folder structure in the ~/Library/Mail/V2 folder. If the update is successful, you shouldn't need anything in this folder, except perhaps the contents of the Bundles folder if you want to re-install any plug-ins.