Now that you have been working with Exchange 2007 for a while, you may notice that in the Exchange Management Console, there are a few new things that you see.  One of these is that when you look at objects, you now see a column labeled Recipient Type Details.  This is, in fact, a new attribute that has been added for Exchange 2007.  There is another attribute called Recipient Display Type.  You will not see them with this name in AD, however.  The mapping is as follows:

Recipient Display Type = msExchRecipientDisplayType
Recipient Type Details = msExchRecipientTypeDetails

These two attributes are new attributes that are added to the Active Directory Schema during Exchange 2007 setup. They are used in determining the type of recipient in the Address Book. They are displayed in the Exchange Management Console under Recipient Configuration, and in Exchange Management Shell if you perform a get- command and instruct the shell to return the full list (i.e. Get-Mailbox | fl )

Like many other Active Directory Attributes, these are represented by Integer Values in Active Directory.  One important item to note here is that you do not want to be arbitrarily changing these values.  The Recipient Display Type is something that should only be set by Exchange.  Changing it via any other method is not supported.

I've included a couple of tables for reference.  The first table covers the different values that can be assigned to the Recipient Display Type attribute.

This second table is perhaps of more interest.  In most situations, you will see objects appear as they should.  For example, a new mailbox (for an existing user or a new user) should always show up as a User Mailbox.  If your Exchange 2007 server co-exists with Exchange 2003 or Exchange 2000 servers, then all mailboxes on those 2000 or 2003 servers will show up as Legacy mailboxes.  You may, however, find under certain circumstances that it does not appear correctly.  For instance, after moving a mailbox from an Exchange 2003 server, which appears as Legacy Mailbox, to your Exchange 2007 server, it may now appear as a Linked Mailbox.  If this happens, you need to investiage why.  One possible cause could be if Associated External Account was set on the 2003 mailbox.

Also note that this attribute will not always be stamped with a value.  For example, Groups do not appear to have this attribute set with a value.  This leads me to conclude that Exchange is able to read this information from other attribute values as well (such as perhaps the GroupType attribute).

Although you should not be changing these values in a production environment, it is always fun to play around in a lab and learn more about what they do.  But make sure you do your playing in a lab environment!

[Update]
Although I couldn't find it previously, some of this information is documented in the Technet Exchange 2007 documentation.  If you happen to be experiencing an issue where a mailbox is showing up with the wrong Recipient Display Type, please reference the following links for details on how to correct that.  Also note that in most cases, there is a reason for a mailbox showing up the way it is.  For example, if you perform a cross-org mailbox move without first migrating the user account, it will show up as a Linked mailbox, and that is normal.  What you don't want to do is simply modify this value without addressing the other reasons for the current value being stamped.

http://technet.microsoft.com/en-us/library/bb201749.aspx - How to Convert a mailbox
http://technet.microsoft.com/en-us/library/bb201694.aspx - How to Convert a mailbox to a Linked mailbox

[thanks to the comment of a visitor, it looks like I was off in my decimal conversions.  I've edited the top table accordingly]