Outlook Increases 500 Shared Folder Limit to 5000

How Outlook 2003 Changed the World of Email Clients

Outlook 2003 introduced “drizzle-mode” synchronization. When Outlook is configured in cached Exchange mode, drizzle-mode synchronization uses a set of background threads to monitor changes in all non-system folders and download changes as they occur. The user doesn’t have to do anything to update the cached (offline) copy of their mailbox. Since the introduction of drizzle mode, Outlook users are accustomed to being able to keep a complete copy of their mailbox for offline access (or a subset of the mailbox as adjusted by the Outlook “slider”).

When Microsoft introduced Outlook 2003, they also included a bunch of network enhancements to make drizzle mode synchronization work smoothly, including high-priority threads to download new messages to the Inbox and upload outgoing messages as they were sent. At a time when abundant network resources exist, it’s hard to look back to a point when synchronization involved many slow dial-up connections and VPNs to emphasize just how good it was to have an efficient way to have a complete offline copy of a mailbox. Outlook 2003 revolutionized the way people worked and laid the foundation for Outlook to be the predominant client for Exchange. Cached Exchange mode rapidly became the de facto standard working model for Outlook and all was well in the world of email.

The Slight Problem of Shared Folders

Except, that is, for shared folders. Drizzle mode synchronization works extremely well for folders in primary mailboxes, but not in secondary mailboxes, such as shared mailboxes or when delegates had access to other peoples’ mailboxes. The classic use case is where an administrative assistant has access to other mailboxes to be able to process inbound messages. In some deployments, I have known assistants working with the mailboxes of over twenty people – and sometimes they weren’t very happy.

Things usually worked OK if Outlook had to cope with just a few shared folders, but problems lurking in the background soon became apparent as the number of folders increased. Items seemed to be missing and performance degraded rapidly. It wasn’t a good situation.

The Outlook and Exchange development teams have been aware of the issue for years, but their understanding of how to track changes in shared folders while respecting permissions to those folders (an issue that doesn’t occur for folders in the primary mailbox) led to a point where Outlook could support a maximum of 500 shared folders (a MAPI restriction: Outlook is still very much a MAPI client).

A New Approach

The good news is that Microsoft has come up with a new approach that will raise the limit from 500. As explained in a June 4 blog, instead of keeping individual shared folders open in memory (which is where the MAPI restriction comes from), Outlook will monitor a MAPI property for the folder that changes when something inside the folder changes (like a new message or the deletion of a message). Once Outlook sees that the property has changed, it can launch synchronization to make sure that the offline copy of the shared folder matches what’s on the server.

The reason why this approach is better is that Outlook doesn’t have to keep folders open to know when changes occur. Memory usage is lower and synchronization should be smoother. Microsoft says that they expect most customers to see the limit increase from 500 to 5,000 folders. They didn’t give any details about what they mean by “most customers” or how users can track how many shared folders Outlook can access.

Changes Available Now

Microsoft has already released these changes in Office ProPlus (click to run) for Office 365, saying: “These changes were released to our Monthly Channel (Targeted) customers  with the April 1904 release, to our Monthly Channel customers with 1905 (11629.20196) and later, and will be coming to our Semi-Annual channel customers on the regular SA schedule (September for Targeted and January for general release.)

To check your version, go to File and then Office Account. As you can see in Figure 1, I currently run build 11620.20214, a later build than 11629.20196, so I have the updated code.

Outlook ProPlus reveals its build information
Figure 1: Outlook ProPlus reveals its build information

No New for Other Outlook Versions

Microsoft hasn’t said if they will update other versions of Outlook, including Outlook 2019, to take advantage of the new approach to synchronizing shared folders. For the moment, this change is restricted to Office ProPlus.

Need more information about Office 365 clients? Look no further than the Clients chapter in the Office 365 for IT Pros eBook.

6 Replies to “Outlook Increases 500 Shared Folder Limit to 5000”

  1. Great news! Does Exchange need to be Office 365 or is it just the click-to-run client? Could this update also apply to Exchange on-premises with Microsoft Office 365 ProPlus as the client?

    1. Click to run, I think. It should work with on-premises servers too as it’s a client-side method of monitoring server folders.

  2. Is there a plan for when this will be introduced in to the business version of O365?

  3. Does this only apply to enterprise editions of Outlook 365 or MS Business Standard or no further information on this yet? 🙂

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