What format does Apple Mail store its emails in? We're trying to convert a user's email mailbox from Apple Mail to Outlook 2010.  

We copied the files off the mac, and they are mostly directories that end in .mbox and contain a folder called Messages containing .emlx files.

The .mbox extension on the folders initially made me think that these messages were in the mbox format, but after some research I found that the mbox format is actually a single file, so that rules out it being in that format.
You can find a description of the format based on an reengineering effort here:

It has three parts:

The length of part 2, in bytes
The message itself
Message metadata (XML Property List)