Fix Outlook on the Web ErrorMailBoxNotFound for Shared Mailboxes
Question details
Users are unable to open shared mailboxes in Outlook on the web due to an ErrorMailBoxNotFound or UserHasNoMailboxException error.
- Product
- Outlook on the web (Microsoft 365)
- Device & OS
- not provided
- Scenario
- Attempting to access a shared mailbox through the Outlook web interface.
- Observed behavior
- The system throws an HTTP 500 error with the message Microsoft.Exchange.Data.Storage.UserHasNoMailboxException and ErrorMailBoxNotFound.
Verify your account permissions to ensure you have been granted access to the shared mailbox, and confirm whether you have administrative privileges in your Microsoft 365 tenant.
Submit a Service Request in the Microsoft 365 Admin Center
Since this is an Exchange Online backend issue, resolving it requires administrative access to contact Microsoft Support for server-side troubleshooting.
The ErrorMailBoxNotFound error in Outlook on the web points directly to a Microsoft Exchange backend synchronization or configuration failure. Because typical end-users do not have the required permissions to modify Exchange server configurations, this issue must be escalated to Microsoft Support via the Admin Center.
Navigate to admin.microsoft.com and log in using an account with Microsoft 365 Administrator privileges.
Locate the 'Support' section on the left-hand navigation menu and click on 'Help & support'.
Type 'ErrorMailBoxNotFound in Outlook on the web for shared mailbox' into the search field and review the automated diagnostics.
If the automated diagnostics do not resolve the issue, select the option to contact support, provide the affected shared mailbox address, and submit the ticket for Microsoft engineers to investigate.
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Frequently Asked Questions
What causes the ErrorMailBoxNotFound error in Outlook on the web?
This error is typically caused by an Exchange Online backend synchronization issue, incorrect mailbox routing, or a delay in permission propagation immediately after a shared mailbox is created or modified.
Can I fix the UserHasNoMailboxException without admin rights?
No. This specific HTTP 500 error indicates a server-side configuration issue that requires Microsoft 365 tenant administrator permissions to resolve through an official Microsoft support ticket.
Does this error affect the Outlook desktop application as well?
While the ErrorMailBoxNotFound specifically triggers on Outlook on the web, underlying Exchange backend issues might also prevent the shared mailbox from updating, synchronizing, or connecting properly in the desktop client.
