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 There were a couple of outdated bundles, which seems logical.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.
Best Answer
The lost+found folder is a special UNIX folder created for a variety of reasons: Most often the lost+found folder is created after a forced shutdown or a power cut. The OS loses track of what was going on and instead of dumping any files or folders it has lost track of, it puts them in the lost+found folder.
It's not much to be concerned about , seeing as those are some old files they can probably be safely deleted.