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Lifting the 500 Folder Limit in Outlook

Feature deep dive
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Hi – I’m Jon LeCroy from the Outlook team, and today I have a story to tell you about the most requested and most rejected (sorry) feature in the history of Outlook.

The limit of 500 shared folders has been a longstanding product limitation in Outlook for Windows since the dawn of cached mode (2003!) For those with access to mailboxes containing more than 500 folders, sync would “randomly” fail resulting in an inconsistent sync for both the customer’s own mailbox, as well as the other mailboxes they had access to. This has been especially painful for those in roles supporting executives. If the executive has many folders, synchronization becomes unreliable for the person they rely on most.

Over the years, many customers have asked us to address this limitation, and understandably so given the impact. Unfortunately, when we looked at this we would find that it was a multi-year engineering effort to complete the changes necessary to rearchitect Outlook to avoid this.

Last fall, several customers each escalated this issue to us again, all at once. This prompted us to take a hard look at the feasibility of addressing this. Was this something fundamental to the design of Outlook and Exchange? Or was there was something we could do about it? Skipping forward to the obvious: we were able to find a way to fix this that avoids the years of work we believed this to take previously.

Technical Breakdown

For the more technical readers out there, I’ll indulge you with some additional detail. Exchange allows for what we call “deep hierarchy” notifications, these will notify the client if something changes within a given folder subtree. For the primary account mailbox, Outlook uses this at the root/top of the mailbox folders to do regular sync. We register the notification, Exchange tells us when something changes, and we sync down the changes.

For shared mailboxes however, this gets trickier. It’s prohibitively expensive (performance wise) in Exchange to evaluate access on a change by change basis to determine if the registered client has access to the item changed, thus deep hierarchy notifications are only available if you’re the mailbox owner. Until now, Outlook would keep each individual folder open and register for changes in that folder (“shallow” notifications) to learn when something changed. Pairing this behavior with Exchange’s default limits of 500 MAPI objects open at a time, and we have the 500 folder limit.

Fortunately, when we looked harder at our options we were able to find another means of learning that something changed in the folder hierarchy – a special property on the folder that is updated when anything within it changes. Now we watch for changes to this property on the shared mailbox, and when detected we’ll work through the shared mailbox to sync any changes. Voilà! Sync for shared mailboxes without running into the 500 folder limit!

So, what’s the new limit?

Since we’ve changed how shared mailbox sync works in a way that works smarter under the previous limitations (and didn’t simply raise the limit in Exchange) I don’t have a simple “new limit” to share with you. The new functional limit will be influenced by Outlook’s available resources and configuration. I am happy to report however that we expect a 10x increase (500 to 5000) for most customers!

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.)


Thank you for reading and for your participation in the Insider program! If you have any issues in Outlook (with sync or anything else), please let us know by going to Help > Contact Support and we’ll be happy to help. Learn what other information you should include in your feedback to ensure  it’s actionable and reaches the right people. We’re excited to hear from you! 

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