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Microsoft is raising the 500 shared folder limit in Outlook for Windows to 5,000

Microsoft has announced that it is finally lifting the limit of 500 shared folders in Outlook for Windows and expects to increase that ceiling to 5,000 for the majority of customers. The company admits that the previous limitation has been a bottleneck especially for those who have access to Outlook containing more than 500 folders because syncing would randomly fail.

The software giant says customers have long been clamoring for the removal of that limitation and after the latest round of request last fall, Microsoft finally decided to take action. Explaining how the limit has been lifted, Outlook's Jon LeCroy explained in a blog post that Outlook uses Microsoft Exchange's deep hierarchy notifications to perform regular syncing. This notification method notifies the client when changes are made in a certain folder subtree, but the method becomes more complicated with shared folders. LeCroy wrote:

"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!"

The update is now available to the Monthly Channel (targeted) customers on the 1904 release of Outlook and to the Monthly Channel customers on the 1905 version (11629.20196) or later. The Semi-Annual channel customers on the regular SA schedule (September for Targeted and January for general release) will see the changes at a later date.

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