For a while, I've had a few "undeletable Outlook folders". Even after deleting all the messages from them, Outlook just complains when I try to delete them. There was some sort of error message, but of course I didn't read it. The only option was OK, so I clicked it. As I recall, the message said something about "Can't delete because blah blah pending synchronization blah blah." I don't know what "pending synchronization" is, but it must be important if Outlook won't let me delete a folder because of it.

Meanwhile, I also noticed that my Sync Issues folder grew by about a dozen error messages every day, and I had to go clean them out every so often. The messages looked something like this:

9:26:59 Synchronizer Version 11.0.6352
9:26:59 Synchronizing Mailbox 'Raymond Chen'
9:26:59 Synchronizing local changes in folder '0618'
9:26:59 Error synchronizing folder
9:26:59		 [80004005-501-4B9-0]
9:26:59		 The client operation failed.
9:26:59		 Microsoft Exchange Server Information Store
9:26:59		 For more information on this failure, click the URL below:
9:26:59		 http://www.microsoft.com/support/prodredirect/outlook2000_us.asp?err=80004005-501-4b9-0
9:27:00 Synchronizing local changes in folder '0611'
9:27:00 Error synchronizing folder
9:27:00		 [80004005-501-4B9-0]
9:27:00		 The client operation failed.
9:27:00		 Microsoft Exchange Server Information Store
9:27:00		 For more information on this failure, click the URL below:
9:27:00		 http://www.microsoft.com/support/prodredirect/outlook2000_us.asp?err=80004005-501-4b9-0
9:27:01 Done

I clicked the link to obtain further information, but the instructions there didn't solve my problem. I just chalked this up to "Outlook gets that way sometimes," and ignored the messages, since they didn't seem to be hurting me. I had almost resigned myself to carrying these two undeletable folders with me until I die.

Then today I randomly stumbled across the solution.

I right-clicked one of the "stuck" folders and selected "Clear Offline Items", even though there were no offline items in the folder. (I deleted all the messages from it; the folder was empty. How do you clear something that is empty?) I got a warning dialog that said something like, "Hey, there's some unfinished synchronization here, do you want to clear the items anyway?" I said, "Go for it."

And then Outlook let me delete the folder.

My guess is that Outlook's synchronization engine got wedged up on these two folders because there was some unfinished business that it just couldn't reconcile, and it said, "Eh, maybe it'll work tomorrow, but in the meantime, don't delete it yet. I'm still working on it." Repeat for several months, because tomorrow never comes. By telling Outlook, "Oh just give up already, trust me, I don't care any more," the synchronization engine released its objections to deleting the folder and let me finally wipe it out.

If you have a mysteriously undeletable folder, you could try this, see if it helps.

Update: I just hit the problem again. The error message is

Outlook hat die Synchronisierung der lokalen Änderungen an Elementen in diesem Ordner noch nicht abgeschlossen. Der Order kann erst nach Abschluss der Synchronisierung mit dem Server gelöscht werden.

Yes, I run Outlook in German. This translates to "Outlook has not yet completed the synchronization of local changes to items in this folder. The folder can only be deleted after completion of the synchronization with the server."

This time, deleting the offline items wasn't good enough. Even though the Properties dialog says "Server folder contains: 0 items" and "Offline folder contains: 0 items", I nevertheless had to trigger a manual synchronization to reconfirm that zero equals zero before it would let me delete the folder.

(Watch, people are now going to start sending me Outlook product support questions. Hey, I don't work on Outlook. I'm a hapless victim like you!)