Error: TimeoutErrorTransientException.

The error you are encountering, TimeoutErrorTransientException, indicates that the migration process is timing out when attempting to send a request to the specified endpoint. To resolve this issue, you can increase the SendTimeout value on the binding configuration used in your Exchange setup. This adjustment allows more time for the operation to complete before timing out.

Here are the steps you can follow to modify the timeout settings:

1. Locate the Web.config file on your Client Access server. For Exchange Server 2010, it is typically found in the following directory: \Program Files\Microsoft\Exchange Server\V14\ClientAccess\exchweb\ews.
2. In the Web.config file, look for the binding settings related to MRSProxyHttpsBinding and MRSProxyWSSecurityBinding.
3. Change the SendTimeout value to a higher duration, for example, from the default to a value like 00:02:00 (2 minutes).
4. Save the changes to the Web.config file.
5. Restart IIS using the iisreset command to apply the changes.

These adjustments should help in mitigating the timeout issues during the migration process.

Hi @Srijith Jayarajan

Thank you for posting your question in Microsoft Q&A. 

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As your description, the respond mesage " TimeoutErrorTransientException" typically occurs during hybrid mailbox migrations from on-premises Exchange to Exchange Online when there is a misconfiguration or connectivity issue with the Mailbox Replication Service (MRS) Proxy endpoint on the on-premises Exchange server.. Common causes include network interruptions, authentication issues, configuration mismatches, or timeouts that prevent Exchange Online from successfully communicating with the EWS/MRSProxy.svc service.

Your on-premises Exchange Server version 15.2.2562.17 corresponds to the Release to Manufacturing (RTM) build of Microsoft Exchange Server Subscription Edition (SE), released in mid-2025. This version is fully supported for hybrid deployments with Exchange Online, so the issue is likely related to setup or environment-specific factors rather than version compatibility.

Could you try to follow these steps to troubleshoot it: 

Verify MRS Proxy is Enabled and Configured:  

Run:

 Get-WebServicesVirtualDirectory | FL Server,Name,MRSProxyEnabled,ExternalUrl,InternalUrl 

Ensure MRSProxyEnabled is True on the relevant server(s). If not, enable it:

Set-WebServicesVirtualDirectory -Identity "ServerName\EWS (Default Web Site)" -MRSProxyEnabled $true 

Confirm the ExternalUrl matches your public endpoint and is resolvable/accessible from the internet. 

Restart IIS:

iisreset /noforce 

Test Connectivity from Exchange Online:  

Connect to Exchange Online .

Run:

Test-MigrationServerAvailability -ExchangeRemoteMove -RemoteServer 'Your sever' -Credentials (Get-Credential) 

If it fails with a similar timeout, note the exact error. This isolates if the issue is inbound connectivity. 

Check firewall rules:

Ensure inbound TCP/443 is open from Exchange Online IP ranges (find current ranges via Get-ServiceIpAddresses in EXO PowerShell or Microsoft's docs). Review firewall/load balancer logs for blocks. 

Check Authentication Settings:  

On the on-premises server, open IIS Manager -> Default Web Site -> EWS -> Authentication. Ensure Windows Authentication is enabled and providers include NTLM/Negotiate. 

Review Event Viewer (Windows Logs -> Security) for login failures (such as bad password or unknown user) around migration times. 

Adjust NTLM level: Open Local Security Policy -> Local Policies -> Security Options -> "Network security: LAN Manager authentication level" and set to "Send NTLMv2 response only" if it's stricter. Restart the server if changed. 

Verify certificate: Ensure a valid, trusted SSL cert is bound to the site (no self-signed for production). 

Increase Timeout Values:  

Edit %ExchangeInstallPath%\ClientAccess\exchweb\EWS\web.config (backup first). 

Under <system.web>, add or modify: <httpRuntime requestValidationMode="2.0" maxRequestLength="65536" executionTimeout="3600" /> (3600 seconds = 1 hour). 

In IIS Manager -> Default Web Site -> EWS > .NET Request Throttling, increase limits if needed. 

For bindings: In IIS > Sites > Default Web Site > Bindings > Edit HTTPS > Increase SendTimeout/ReceiveTimeout if using custom bindings. 

Restart IIS after changes and check if the issue still persists. 

This summary is based on my findings from the community and several relevant threads. However, it may not accurately reflect the behavior in question. To help you reach your goal more effectively, I recommend posting a thread on the Microsoft Tech Community forum. It’s a great platform for deeper technical discussions and connecting with individuals who have hands-on experience and expertise. They’re best positioned to provide guidance and valuable insights on this topic.    

Apologies for redirecting you to the related development team support as the moderators in this community have limited resources to check the backend information, and to get the fast and better assistance we requested for it.   

I hope this helps.

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