We are an Office 365 customer and about two weeks ago received reports of users (with Hosted Exchange mail) who were previously working correctly but spontaneously had "Updating local" displayed in the Outlook activity/status in the lower portion of the Outlook window.  These users were no longer receiving new email in their Inbox as if the (apparent) single thread that Outlook appears to use to synchronize the Inbox was busy "Updating local".  For many of these situations "Updating local" either eventually went away after a longish period of time (possibly up to two hours) or we told the user to "Empty" the Inbox cache thus forcing a redownload of their entire Inbox.

My curiosities:

1) Why did this start happening?  There was no software change on the client side and although I'm sure enormous Inboxes are adding to the pain, all of these users didn't cross the same size/item count threshold at the same time.

2) Are there any mitigation options?  Most users who I've walked through Emptying their Inbox cache seem ok, but would it be better/faster to tell them to wait for "Updating local" to finish?

Thanks,

- Patrick