Your Exchange Server Administrator has blocked the version of Outlook that you are using

I had a problem recently that was driving me crazy. It was on a migration from Exchange 2003 to Exchange Server 2007. The first sympton was that all of the Outlook clients were getting the message:
"Your Exchange Server administrator has blocked the version of Outlook that you are using."

Of course, I checked the Registry on the E2K7 server and there was no client version being blocked. I read through the Microsoft KB article "When you use Outlook with an Exchange 2007 mailbox, you cannot connect to Exchange 2007, and you receive an error message " but everything was good on my configuration.

I was having a separate problem that I thought was unreleated, though. The public folder hierarchy was not replicating to the Exchange Server 2007 public folder database. Anytime I tried to use the *-PublicFolder* cmdlets, I got the below error:

There is more than one MAPI public folder tree in administrative group. This may cause access denied errors when accessing public folders

I went through the KB article: "Multiple MAPI public folder trees detected" but it was no help. Well, first of all, it was never possible to create more than one MAPI public folder tree in Exchange 2003, but still, I checked on the E2K3 servers for more than on public folder tree (of any kind). I drilled down via ADSIEDIT to the Public Folder container but did not find any other public folder tree other than the default one.

Finally, I searched the entire configuration Container for public folder trees and found a CNF (conflict) object directly under the Exchange organization object in the Configuration container. The object was very old so I'm assuming that at some point, when this Exchange organization was migrated from Exchange 5.5 to Exchange 2003, the Active Directory Connector was misconfigured and it replicated the public folder tree twice.

Anyway, once I deleted this CNF public folder tree object, both problems went away, the public folder hierarchy and folder content started replicating, and life was good!