Outlook.com mailbox inaccessible across web and apps — HTTP 500 / StartupData / ServerError

Question

Outlook.com mailbox inaccessible across web and apps — HTTP 500 / StartupData / ServerError

Frank C Su 0

Since early morning on 18 Sep 2026 (HKT), my personal Outlook.com mailbox has stopped synchronizing. The last successfully synchronized email was around 3:45 AM HKT.

The issue affects multiple clients and devices:

Outlook Web cannot load the mailbox
Outlook mobile app cannot refresh or display new emails
Outlook desktop also cannot synchronize
iOS push notifications for new emails are still arriving
Other Microsoft services on the same account, such as OneDrive, are working normally

Outlook Web fails during mailbox initialization with:

BootResult: fail
Error: 500
esrc: StartupData
et: ServerError
st: 500

This appears to be a mailbox/backend issue rather than a browser, device, network, or sign-in problem.

I am unable to use Outlook in-app support because Outlook itself cannot load. Please investigate whether this is affecting a subset of Outlook.com mailboxes or backend infrastructure.

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Answer 1

Error like this is always server side.

Can you please try this link in incognito mode and see if it works: https://outlook.office365.com/mail/

There is currently an incident. I will share the information here:

Users are unable to access Outlook on the web from a specific Outlook link

Issue ID: EX1473059

Affected services: Exchange Online

Status: Service degradation

Issue type: Advisory

Start time: Sep 15, 2026, 10:00 AM CDT

User impact

Users are unable to access Outlook on the web from a specific Outlook link.

More info

Users are unable to access Outlook on the web from a specific link within Outlook.

Users can access Outlook on the web by navigating to a direct URL with /mail/ appended.

As an example, some users can navigate to "https://outlook.office365.com/mail/" to

successfully access the client.

Department of Defense customers

https://webmail.apps.mil/mail/

GCCH customers

https://outlook.office365.us/mail/

Scope of impact

This issue may prevent any user attempting to access Outlook on the web from the specific link in the More Info portion of this communication. This information may be updated as our investigation continues.

Root cause

A recent code change is producing a code regression that is preventing users from accessing Outlook on the web from a specific link within Outlook.

Current status

Sep 17, 2026, 1:47 PM CDT

Our change reversion is approximately 82 percent complete, and our established timeline expects it will have completed by our next scheduled communication update.‌

Next update by:

Friday, September 18, 2026 at 4:00 PM CDT