What are the .mbox files in ~/Library/Mail/Mail Lost+Found/Mailboxes in Mail.app 5.0 (Lion)?

When I upgraded to Lion, Mail.app 5.0 created a directory ~/Library/Mail/Mail Lost+Found. There were a couple of outdated bundles, which seems logical. But there is also a Mailboxes directory that has a lot (~30-40) .mbox files. Here is the contents of one of those files:

total used in directory 224 available 282057236
drwxr-xr-x   6 kmm  kmm     204 Aug  1 15:55 .
drwxr-xr-x  17 kmm  kmm     578 Aug  2 08:47 ..
-rw-rw-r--   1 kmm  kmm       0 Nov  6  2007 .index.ready
-rw-rw-r--   1 kmm  kmm  109793 Nov  6  2007 Incoming_Mail
-rw-rw-r--   1 kmm  kmm    1860 Nov  6  2007 Incoming_Table_of_Contents
-rw-rw-r--   1 kmm  kmm       0 Nov  6  2007 mbox.SKindex.isValid

Is it safe to delete these files? I haven't opened them all to determine if any contains actual messages. The few I have opened have not.

They only occupy 9.5 mb in total, which isn't much, admittedly. But I have something against cruft hanging around when it's not needed.

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.