Users may encounter a session mismatch error when accessing a shared mailbox in the new Outlook desktop application. This article explains why the error occurs and how to resolve it.
Symptoms
You may experience one or more of the following:
A Session Mismatch error when opening or syncing a shared mailbox.
The shared mailbox does not load correctly in ExtendSync.
Authentication or synchronization issues affecting only the shared mailbox.
Why This Happens
This issue occurs when the same mailbox is configured in two different ways:
As a standalone Outlook account
As a shared mailbox
When both configurations exist simultaneously, Outlook maintains separate authentication sessions for the same mailbox, resulting in a session conflict.
This is a mailbox configuration issue and is not caused by ExtendSync.
How to Resolve the Issue
End Users
Remove the shared mailbox if it has been added as a standalone Outlook account.
Open New Outlook.
Select File.
Go to Accounts > Your accounts.
Locate the shared mailbox.
Select Manage next to the shared mailbox.
Click Remove account (or Remove) and confirm the removal.
Restart Outlook.
Verify that the mailbox is available only as a shared mailbox.
Important: Only the standalone account should be removed. The shared mailbox should remain configured as a shared mailbox.
Administrators
If multiple users report the same error:
Verify that the affected mailbox has not been added as both a standalone account and a shared mailbox.
Remove the standalone account configuration for affected users.
Ensure users access the mailbox only through the shared mailbox configuration.
✅ Verification
The issue is resolved when:
The shared mailbox opens normally.
No Session Mismatch error is displayed.
ExtendSync successfully accesses the shared mailbox.
Outlook shows the mailbox only once, as a shared mailbox.
Need Help?
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If you still need assistance:
💬 Use in-app chat (bottom-right of the support site)
✉️ Email cloudextend-support@celigo.com