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How to Fix Mailbox Migration Failures from Exchange to Exchange Online
Question details
Users are experiencing failures when migrating several mailboxes from an on-premises Exchange organization to Exchange Online within a hybrid deployment.
- Product
- Microsoft Exchange Online
- Device & OS
- not provided
- Scenario
- Moving mailboxes to the cloud in an Exchange hybrid deployment environment.
- Observed behavior
- Several mailboxes fail to migrate successfully to Exchange Online and generate migration errors in the batch report.
Ensure you have Exchange Administrator permissions and verify that your on-premises Exchange server has a stable internet connection to Microsoft 365 migration endpoints.
Verify Migration Endpoints, Permissions, and Connectivity
Check the core configuration requirements for hybrid mailbox migrations to resolve common endpoint and connectivity issues.
Most mailbox migration failures in a hybrid setup are caused by incorrect endpoint configurations, insufficient administrator permissions, or firewalls blocking connectivity to Microsoft 365.
Go to the Exchange admin center (EAC), navigate to Migration, and check the migration endpoints to ensure the on-premises server details are accurate and fully reachable.
Ensure the administrator account used for executing the migration has both the 'Recipient Management' and 'Organization Management' roles properly assigned.
Use the Microsoft Remote Connectivity Analyzer tool online to test Exchange Server connectivity and verify that local firewalls are not blocking Microsoft 365 IP addresses or necessary ports.
Contact Microsoft 365 Support with Error Details
If manual troubleshooting does not resolve the migration failures, export the logs and escalate the issue to Microsoft Support.
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Frequently Asked Questions
Why do mailbox migrations pause or fail at 95% in an Exchange hybrid setup?
Migrations often pause or fail at 95% because they are waiting for the final synchronization and mailbox switchover. This can occur if the target delivery domain is incorrectly configured, the migration batch is set to manual completion, or there are unexpected backend connectivity drops.
How can I view detailed error messages for individual failed mailboxes?
You can view detailed errors by navigating to the Exchange admin center, going to Recipients > Migration, selecting the migration batch in question, clicking 'View details', and then choosing the specific user to review the 'Skipped item details' or error summary.
Do I need to assign Microsoft 365 licenses before migrating mailboxes?
Yes, you must ensure that the target user account in Microsoft 365 has a valid Exchange Online license assigned before the migration completes. If the user is unlicensed, the migration process will fail to properly provision the cloud mailbox.
