Outlook.com mailbox returning TooManyObjectsOpenedException at StartupData — persists 6+ hours across datacenters

Question

My Outlook.com account cannot be opened in any browser or in the new Outlook for Windows client. Sign-in succeeds, then the mailbox fails to load with a 500 and the error below. This has persisted for over six hours.

err: Microsoft.Exchange.Data.Storage.TooManyObjectsOpenedException

esrc: StartupData

BootResult: throttle

st: 500

ehk: X-OWA-Error

Three captures, all identical failures:

1. 2026-10-12T18:05:03Z — Session Id dcd96f7e-1f7a-4c77-a8c6-49937ab1a60a — reqid b252e462-535b-b182-12f2-59e9b8dab43e — efe BN9PR03CA0937 — client 20261002003.12
2. 2026-10-12T19:59:07Z — Session Id 63329049-2c2c-43c3-824e-19f3d9740123 — reqid 3b52f8a1-a0f3-2ac3-30e1-404df1739289 — efe BN9PR03CA0846 — client 20261002003.12
3. 2026-10-12T23:26:43Z — Session Id 28e9d1a2-a69e-4ce5-90be-a0fdb984f7fb — reqid a41d9d42-83b7-371a-012f-238618e275f6 — efe BN1PR13CA0010 — client 20261002003.13

ewsver: 15.21.472.18 on all three.

What I have already established:

• Reproduces in a clean incognito profile with no cached cookies or session state.
• Reproduces in the new Outlook for Windows desktop client, which reports the generic "We couldn't reach the email server" message.
• Failures occurred across two different front-end datacenters (BN9 and BN1) and two different client builds, so this is not a single bad server or a stale client bundle.
• The mailbox was left completely idle for over 90 minutes between the first and second attempt, and over three hours before the third. The session budget is not recovering.
• The Outlook mobile app, holding a session established before this started, still syncs normally and successfully sent a message today. So the mailbox data is intact, the account is in good standing, and outbound mail works.

That last point is why I believe this is a server-side session-state issue on my mailbox rather than anything client-side. Authentication is fine; the failure is specifically at StartupData when a new client tries to attach.

I have not removed the account from the mobile app, since that session is currently my only access.

Could someone look at the mailbox itself? Standard client-side troubleshooting (reinstall, clearing credentials, removing and re-adding the account, different browsers, different networks) does not apply here and I would rather not churn more sessions against a mailbox that appears to need to go quiet.

Answer 1

This requires Microsoft Support to investigate the Outlook.com mailbox service; submit a case through Microsoft Get Help and provide the error plus the UTC timestamps, request IDs, session IDs, client versions, and cross-client reproduction details privately.

Open a support case

1. Open Microsoft Get Help while signed in with the affected Microsoft account.
2. Enter: Outlook.com mailbox fails during StartupData with TooManyObjectsOpenedException and HTTP 500.
3. If self-help is offered, continue until the contact/support option appears.
4. Ask for escalation to the Outlook.com mailbox service team and include:
   • Microsoft.Exchange.Data.Storage.TooManyObjectsOpenedException
   • esrc: StartupData
   • BootResult: throttle
   • st: 500
   • Each UTC occurrence, request ID, session ID, client version, and front-end identifier
   • Reproduction in Outlook.com InPrivate browsing and new Outlook for Windows
   • The fact that an already-established Outlook mobile session continues to work
5. State explicitly that the request is for mailbox-side investigation, not password recovery or desktop-client troubleshooting.

Microsoft’s documented Outlook.com support flow normally starts at Outlook.com → Help → Help → Search Help → Still need help? → Yes, but that route is impractical when mailbox startup itself fails. The same support article provides Microsoft Get Help for other Microsoft products and services.^1^

Microsoft-hosted Q&A guidance for comparable persistent StartupData/500 failures recommends giving Support the UTC time and correlation identifiers so the mailbox can be investigated; Q&A participants cannot access or modify the backend mailbox.^2^

Preserve the working mobile session

Do not remove the account, sign it out, reset it, or delete its sync partnership while it remains the only working access path. None of those actions repairs a mailbox-side startup failure, and reauthentication could subject mobile to the same failing startup path.

If Get Help does not provide a usable contact option, Outlook mobile has a documented in-app channel:

1. Outlook mobile → Inbox or the account menu.
2. Tap ?.
3. Select Contact Outlook Mobile Support.^1^
4. Before supplying the full diagnostic package, ask whether that team can accept and escalate a mailbox-wide Outlook.com service case. This channel is officially intended for mobile-app issues, so it is a fallback rather than the primary route.

What the evidence establishes

The cross-browser and cross-client reproduction at StartupData, while authentication and an existing mobile session continue working, supports treating this as a mailbox/service-side startup failure rather than repeating cache, credential, reinstall, browser, or network steps. Similar Microsoft Q&A cases involving Outlook.com StartupData failures across devices have been identified as server-side and potentially temporary.^3^

The exact internal cause is not established by the exception alone. Check Microsoft’s Outlook.com known-issues/service-health page for a declared incident, but absence of a listed incident does not eliminate a mailbox-specific failure.^4^

Do not apply registry or Exchange Management Shell advice concerning open-object limits. That documentation applies to customer-managed Exchange Server 2016, 2019, and Subscription Edition, not consumer Outlook.com; only Microsoft can change the hosted service configuration.^5^

---

References

1. How to get support for Outlook.com | Microsoft Support
2. 2 gündür 500 error hatası alıyorum çıldırmak üzeriyim - Microsoft Q&A
3. My Outlook.com mailbox is inaccessible across all clients. - Microsoft Q&A
4. Fixes or workarounds for recent issues on Outlook.com | Microsoft Support
5. Managed Store Limits in Exchange Server