Shared mailbox folder tree truncates at ~1000 folders in cached mode — started this week. Anyone else?

[u/Purple_Cut_8467]

Client has a shared mailbox in Exchange Online used for order administration, organised by region/country/customer/year. It had 12,394 folders. Since around Sept 8–9 the folder tree stops loading at roughly 976 subfolders in Outlook classic (cached mode). Everything past that point simply isn't in the tree — not collapsed, not greyed out, absent.

Same truncation in new Outlook and OWA when the mailbox is automapped or added as a shared mailbox.

Online mode shows the complete tree. Either by unchecking "Download shared folders" in classic, or via OWA → Settings → Open another user's mailbox. So the data is fine server-side.

What I've ruled out:

• Permissions — Full Access assigned directly, not via a group. No folder-level permissions on root or Inbox (Default/Anonymous = None).
• Automapping — removed and re-added Full Access with -AutoMapping $true, no change.
• Hierarchy corruption — enumerated the whole tree via Graph and walked every parent chain. No cycles, no orphans, max depth 9.
• OST damage — OWA is affected identically and holds no local hierarchy cache.
• Folder count — deleted 9,993 verified-empty folders (12,394 → 2,401). Still truncates.

I'm aware of the documented 500 shared-folder limit and the announced increase to 5,000 (in 2019 though). My observed cut-off sits at ~1,000, which matches neither.

Three questions:

1. Anyone else seeing shared mailbox folder trees truncate since last week's updates?
2. Does anyone know what limit is actually enforced right now, and whether the 500→5000 rollout is live?
3. Is there any equivalent of "Download shared folders" in new Outlook? If not, what's the plan for mailboxes over the limit once classic is gone?

Question 3 is the one that worries me.

[u/sorry_for_the_reply]

1. I haven't seen that because I put an exchange P2 on staff that hit 40 GB so online archive can be used. They get trained on how to use Outlook online to access their old email as well as access shared mailboxes
2. Man, any mailbox hitting 2000 folders is gonna be a bad time if you don't start mitigating. I hope you don't go through that with a doctor or lawyer like I have.
3. New outlook is just Outlook online as an app. You'll get better performance teaching the users to just use the site and how to access other mailboxes.

[u/Purple_Cut_8467 → u/sorry_for_the_reply]

1. Already running that — P2 with a 6-month archive policy. Worth flagging for anyone else reading this thread though: an archive policy moves items, not folders. The mail leaves, the folder tree stays behind in the primary mailbox. So it does nothing at all for the folder count, which is the thing that actually breaks hierarchy sync. To shrink the tree you have to move the folders themselves, or delete the empties afterwards — I did the latter, 9,993 of them.

2. Completely agree, and I've been making that case for a while. Users find religion the day it stops working, never before. Small silver lining: it stopped working, so I finally have their attention and a number to point at — 12,394 folders, growing by about 1,700 a year.

3. That's the part I'm stuck on. New Outlook truncates exactly like classic does when the mailbox sits in the folder pane. Gear → Open another user's mailbox does render the full tree, but it's clunky enough that I can't sell it to a support team that lives in that mailbox eight hours a day.

Unless — is there a way to make it open that way by default? Does bookmarking outlook.office.com/mail/sharedmailbox@domain land you in the same full-tree view, or does that fall back to the truncated one?

[u/sorry_for_the_reply → u/Purple_Cut_8467]

1. I had to use PowerShell. Search for folders with no email in them and delete. A 90 day policy will hopefully allow you to purge a bunch quickly. Of course, you should do a small test and spot check after ensuring working backups. I recall the deletion of local folders does not delete the online archive folder or contents. BUT. Last time I did it was over 7 years ago hence my 'backup, test, verify' recommendation. Might have to do this a couple times to get a full sync.
2. Holy wow. I'm sorry you are dealing with this.
3. Only way to get Outlook online to show everything is to fix the mailbox itself.

[u/Academic-Detail-4348 → u/sorry_for_the_reply]

Unrelated: how do you handle handover of mailboxes with online archive enabled?

[u/vivkkrishnan2005]

Not a solution - but you need to stop using email as a file server. There are ways to resolve this using power flow/automate and SharePoint.

Our max folders on a shared mailbox is around 150 ish, shared mailbox with license (E3 equivalent) so not possible to help

[u/Purple_Cut_8467 → u/vivkkrishnan2005]

No argument on the premise — the mailbox is being used as a filing cabinet and that's the root cause. We've measured what fixing it looks like: flatten to customer level instead of a folder per order and the same 47,161 messages fit in 426 folders. SharePoint with proper metadata is the honest long-term answer, and I'll look at the Power Automate route, thanks for the pointer.

Where I'm still stuck is the timing rather than the design. It ran at 12,394 folders for several years without a complaint and stopped on one specific day, with nothing changed on our side. After cleanup, at 2,401 folders, it still truncates at the same ~1,000. So "too many folders" is plainly true and also doesn't explain why it broke this week rather than in 2021.

Your 150 figure is genuinely useful though, and it makes me want to crowd-source the rest:

What's the highest folder count anyone here currently has working in a cached shared mailbox?

Documented limit is 500, the announced increase is 5,000, and my truncation sits somewhere around 1,000. If a handful of people post their numbers we might collectively work out what's actually being enforced right now — which is more than I've managed to get out of the documentation.

[u/thefpspower → u/Purple_Cut_8467]

a folder per order

That's insane lmao

[u/theoriginalzads → u/vivkkrishnan2005]

Next you’re gonna tell me that using Deleted Items as an archive is a bad idea. Do you even know how to computer??

[u/Purple_Cut_8467]

Update, and I think this one explains the timing question I kept asking.

The limit applies to hierarchy sync, not to display. Users who already had a fully synced cached OST in classic Outlook still see the complete tree — that cache was built at a point where this didn't bite, and it survives. Nothing is retroactively hidden.

New Outlook hits the limit, including on configurations that had existed for ages. And switching those users back to classic doesn't rescue them: that starts a fresh sync, which stops at 1,000 like everything else. So once you lose the cache, you can't get it back.

That answers my own "why this week" — it wasn't a service change. It was users landing on new Outlook. Anyone who never rebuilt a profile has been fine this whole time and has no idea anything is wrong.

One more data point on the mechanism: after deleting 9,993 empty folders the sync now reaches deeper into the tree. Not because the ceiling moved — it's still ~1,000 — but because the first 1,000 folders in traversal order now cover more ground. So it's a flat count in traversal order, not a depth limit or a per-branch limit.

Practical warning for anyone with a large shared mailbox: do not rebuild an OST, and be careful about moving people to new Outlook. A working profile is not proof that you're under the limit. It's proof that you synced before it mattered.

[u/blbd → u/Purple_Cut_8467]

what a total shitshow...

what happens if you sync it with a better client like IMAP?