Email Migration

Fix Microsoft 365 Error: On-Premises Mailbox Has Not Been Migrated

Camila Milosovich Oct 9, 2026 869 views

Question details

The user needs to resolve an error message in Microsoft 365 indicating that an on-premises mailbox has not been migrated, despite the migration being complete.

How to Fix the Microsoft 365 Message That an On-Premises Mailbox Has Not Been Migrated

Product: Microsoft 365
Device & OS: not provided
Scenario: Verifying or completing email migration from an on-premises Exchange server to Microsoft 365.
Observed behavior: Microsoft 365 falsely displays that the mailbox has not been migrated, likely due to directory synchronization issues or stale mailbox attributes in the backend.

Before you start

Ensure you have the exact email addresses of the affected mailboxes and verify that you have Global Administrator privileges in your Microsoft 365 tenant before proceeding.

Solution 1Recommended

Open a Service Request with Microsoft 365 Support

Because this issue involves backend synchronization logs and stale mailbox attributes, a Global Administrator must request assistance directly from Microsoft.

This error is typically a backend synchronization issue that cannot be resolved through standard admin center settings. The Microsoft 365 support team needs to inspect the tenant configuration, specific mailbox attributes, and directory synchronization logs to clear the error state.

Log in to the Admin Center

Navigate to the Microsoft 365 admin center (admin.microsoft.com) and sign in using an account with Global Administrator credentials.

Navigate to Help & Support

Click on the 'Help & support' icon located in the bottom right corner of the admin center dashboard.

Create a Service Request

Describe your issue by stating that an on-premises mailbox shows as not migrated despite completion. Follow the prompts to submit the ticket for backend team review.

Open a Service Request with Microsoft 365 Support

Purchased through a reseller?: If your Microsoft 365 subscription was provided through a reseller, you will not be able to open a ticket directly with Microsoft. You must contact your reseller for technical support.

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Frequently Asked Questions

Why does my mailbox show as not migrated after the batch completes?

This typically happens due to delays in directory synchronization or stale attributes caching in the Microsoft 365 backend. The migration itself might be complete, but the status display fails to update automatically.

Can an end-user fix the 'mailbox not migrated' error?

No, end-users cannot resolve this issue on their own. A Microsoft 365 Global Administrator must investigate the tenant and open a support ticket with Microsoft to fix the backend attributes.

How long does it take for directory synchronization to update mailbox attributes?

Standard directory synchronization (like Azure AD Connect) runs every 30 minutes. If the mailbox status hasn't updated after a few hours and forcing a manual delta sync doesn't work, Microsoft support must intervene.

Migration & Recovery