Each user mailbox is divided into two subtrees:

	* the IPM (Interpersonal Messaging) subtree, which contains
	  the normal, visible folders such as Inbox, Calendar, etc.

	* the non-IPM subtree, which contains internal data,
          preferences, etc

Clients present the root folder of the IPM subtree to the user as
though it was the top of the hierarchy; the non-IPM subtree is
generally completely invisible.

When using WebDAV, the non-IPM subtree can be accessed by appending
/NON_IPM_SUBTREE/ to the end of a mailbox URL. (It is never returned
when listing the subfolders of the mailbox root folder though.)

Outlook creates a number of subfolders in the non-IPM subtree:

	* Views, Common Views - used to store folder view data

	* Deferred Action - server-side rules that use OP_DEFER send
	  messages into this folder. (I think they're just linked from
	  Inbox rather than actually being moved, but I haven't
	  investigated this thoroughly yet.) This is used for rules
	  that are processed partly by Exchange and partly by Outlook.

	* Finder - used as the parent folder for temporary search
	  folders

	* Freebusy Data - contains two files, Sniffer.EML, whose
          purpose is unknown, and FreebusyData.EML, which is discussed
          in the file "freebusy"

	* Net Folder Inbox - this has something to do with the Outlook
          2000 "Net Folders" functionality

	* Reminders - this is essentially a persistent search for the
          next instance of each appointment with an associated
          reminder.

	* Schedule - Not sure what this is for

	* Shortcuts - Stores the contents of the "Favorites" Public
          Folder subtree.

	* Spooler Queue - I assume Outlook uses this when printing?

The NON_IPM_SUBTREE folder also has the useful property that the
"default folders" properties (urn:schemas:httpmail:inbox, etc) are
duplicated there, and can always be read if you have permission to
access the folder they point to. This lets us reliably find other
users' calendars, etc, when they don't have the standard (English)
names.

The Public Folder hierarchy also has a non-IPM subtree
(/public/NON_IPM_SUBTREE), which contains stuff we mostly don't
understand or care about:

	* EFORMS REGISTRY - custom Outlook forms are kept here

	* SCHEDULE+ FREE BUSY - public free/busy data (see "freebusy")

	* OFFLINE ADDRESS BOOK - apparently the source of OAB data.
          Unknown format.

	* Events Root, StoreEvents* - WSS event stuff

	* OWAScratchPad - Presumably OWA temporary space of some sort

	* Nntp Control Folder - Presumably something NNTP-related