Ed,
Not that I know of, MBX/DBX are the message store (extenstion is version dependent), IDX was used in OE4 (and maybe OE3 too). OE5 also can have .LOG files if you have the option checked.
>Dave, aren't there also .PST files associated with OE? It's been a while since I looked, but I thought it used the same type of message archive system that Outlook used.