A shared mailbox used for order administration is organized by region, country, customer, and year, with more than 12,000 folders. Since around September 8–9, the folder tree stops loading at roughly 976–1,000 folders in classic Outlook cached mode. Folders beyond that point are completely absent rather than collapsed or disabled.
The same behavior occurs in new Outlook and in the web client when the mailbox is automapped or added as a shared mailbox. The complete tree is visible when classic Outlook is switched to online mode by disabling Download shared folders, and it is also visible when opening the mailbox through the web client's Open another user's mailbox option. This suggests the server-side data is intact.
Full Access is assigned directly, automapping has been tested, and there are no relevant folder-level permission restrictions. The hierarchy was checked through Graph with no cycles, orphaned folders, or unusual depth. Deleting 9,993 confirmed-empty folders reduced the mailbox from 12,394 to 2,401 folders, but synchronization still stops at approximately the same point.
The documented shared-folder limit is often described as 500, with a previously announced increase to 5,000, but the observed cutoff appears to be around 1,000. Has anyone else seen this after recent Outlook changes? What limit is currently enforced, and does new Outlook have an equivalent to Download shared folders? If not, how are large shared mailboxes expected to work once classic Outlook is no longer available?
3 Answers
The behavior appears to be tied to hierarchy synchronization rather than the mailbox actually losing folders. Existing classic Outlook profiles that had already cached the complete tree can continue showing it, while new Outlook or a newly rebuilt classic profile performs a fresh sync and stops at about 1,000 folders. That explains why the issue can appear suddenly when users move to new Outlook even though the mailbox has existed unchanged for years.
Deleting folders can make the visible portion reach further into the hierarchy because the first 1,000 folders in traversal order now cover more of the tree, but it does not seem to raise the ceiling. Be very careful about rebuilding OST files or moving users to new Outlook if their current cached profile still shows everything.
New Outlook is essentially the web experience packaged as an application, and it does not appear to offer a direct equivalent to classic Outlook’s Download shared folders setting. Opening the mailbox separately through the web client can expose the full hierarchy, but that is awkward for staff who work in the mailbox all day. A bookmarked direct mailbox URL may be worth testing, but it is more of a workaround than a proper fix.
For a mailbox this large, the long-term answer is probably to move away from using folders as a filing system. SharePoint or another document-management workflow with metadata and automation would scale better, even if that does not solve the immediate synchronization problem.
A practical redesign showed why this matters: grouping at the customer level instead of creating a folder for every order reduced the same message set to a few hundred folders. Online archives will not fix the hierarchy count because they move messages, not the folders themselves.
The commonly cited limits do not seem to describe every current synchronization path consistently. The evidence here points to a flat traversal cap of roughly 1,000 folders for a fresh hierarchy sync, rather than a depth limit, branch limit, or permission issue. The fact that the web client can display the complete tree while cached clients cannot is a strong indication that this is a client or service synchronization constraint.
Until Microsoft clarifies the active limit and provides a supported setting in new Outlook, preserve any working cached profiles, avoid unnecessary OST rebuilds, and plan to reduce the folder count or migrate the filing process. IMAP is unlikely to be a reliable substitute for an Exchange shared mailbox because it will not reproduce all shared-mailbox features, permissions, or folder behavior.
That also explains why some users may report no problem: their old cache is still intact. A working profile does not necessarily mean the mailbox is below the current synchronization limit.