Fix Access Denied Errors During Exchange Hybrid Mailbox Migration
Question details
Users encounter Access Denied errors and transient exceptions when migrating mailboxes from on-premises Exchange to Exchange Online.
- Product
- Microsoft Exchange
- Device & OS
- not provided
- Scenario
- Migrating mailboxes from on-premises Exchange servers to Microsoft 365 Exchange Online.
- Observed behavior
- Most mailboxes fail to migrate, returning 'MigrationTransientException' or 'TooManyTransientFailureRetriesPermanentException' because the EWS MRSProxy endpoint returns Access Denied.
Ensure you have Microsoft 365 global administrator privileges before attempting to open a support ticket or manage hybrid server settings.
Open a Microsoft 365 Support Request
Since the issue is often caused by backend resource limitations on the hybrid server, contacting Microsoft Support is the most reliable way to resolve MRSProxy failures.
Backend resource throttling can cause the Mailbox Replication Service (MRS) Proxy endpoint to deny access during large or resource-intensive migrations. Microsoft support can investigate and potentially adjust throttling policies on the backend.
Navigate to the Microsoft 365 admin center using your global administrator credentials.
On the left-hand navigation menu, locate and click on 'Support'.
Select 'New service request' and provide the details of your MigrationTransientException and MRSProxy Access Denied errors for the backend team to investigate.
Retry the Migration Batch
Transient exceptions can sometimes resolve themselves if they are caused by temporary server throttling or resource limits.
Looking for a Lightweight Office Solution After Migration?
While managing complex Exchange migrations and IT infrastructure, you need an office suite that won't slow you down. WPS Office provides a free, lightweight, and highly compatible alternative to Microsoft Office, ensuring seamless productivity without a heavy resource footprint.
- 1. Download the Installer: Visit the official WPS Office website and click the free download button for your operating system.
- 2. Install WPS Office: Run the downloaded installer file and follow the simple on-screen instructions to complete the setup.
- 3. Open Your Documents: Launch WPS Office and instantly open your existing Microsoft Office files with perfect formatting compatibility.
Frequently Asked Questions
What does MigrationTransientException mean during Exchange migration?
This exception indicates a temporary issue, often related to server resource limitations, network timeouts, or throttling on the Microsoft 365 backend. In many cases, retrying the migration later will resolve it.
Why is the EWS MRSProxy endpoint returning Access Denied?
An Access Denied error from the Mailbox Replication Service (MRS) Proxy usually happens when the backend server is overloaded, connection limits are reached, or there is an authentication misconfiguration on the hybrid server.
How can I check if MRSProxy is enabled on my Exchange server?
You can verify the MRSProxy status by opening the Exchange Management Shell and running the 'Get-WebServicesVirtualDirectory' cmdlet. Ensure that the 'MRSProxyEnabled' parameter is set to true for your environment.
