Mailbox Replication from Exchange OnPremise to Exchange Online

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Hi Stefan Arndt

Thank you for posting your question in the Microsoft Q&A forum regarding migration issue affecting your Exchange Online mailbox moves. Based on the analysis of the error message:

The call to https://xxxx/EWS/mrsproxy.svc timed out…

This indicates that the migration process is being interrupted due to a timeout between Exchange Online and your on-premises MRSProxy endpoint. The issue is not related to authentication or certificates, but to how the Mailbox Replication Service (MRS) handles large or long-running transfer sessions particularly over slower upload connections (2 Mbit/s).

Below are the recommended steps you can try to address and stabilize the migration process:

Validate EWS and MRSProxy Connectivity

Run this from Exchange Management Shell on your Exchange 2016 server:

Get-WebServicesVirtualDirectory | fl Identity,InternalUrl,ExternalUrl,MRSProxyEnabled
Test-MigrationServerAvailability -ExchangeRemoteMove -RemoteServer mail.contoso.com -Credentials (Get-Credential contoso\administrator)

• If it fails or hangs, check the Application Event Log > MSExchange Mailbox Replication Service for matching timeout entries.
• Success should show a response time <10000 ms and “Succeeded.”

Troubleshooting Hybrid Migration Endpoints in Classic and Modern Hybrid | Microsoft Community Hub

Increase MRSProxy and EWS Timeouts

First, make sure to back up the file, then go ahead and edit the following:

C:\Program Files\Microsoft\Exchange Server\V15\ClientAccess\exchweb\ews\web.config

Adjust or add the following configuration:

<MRSProxyConfiguration IsEnabled="true" MaxMRSConnections="100" DataImportTimeout="00:20:00" /><httpRuntime executionTimeout="3600" maxRequestLength="512000" />

After saving the changes, run:

iisreset /noforce 

Restart-Service MSExchangeMailboxReplication

This increases the allowed request duration for MRSProxy and prevents premature timeouts during mailbox transfers.

Review NAT or Firewall Timeouts

If the Exchange server is published through NAT, ensure the device does not close idle HTTPS sessions too quickly.

• Increase the TCP session idle timeout to at least 30 minutes.
• Disable any web or TCP proxy inspection for this connection.

A short NAT timeout can close the migration session before completion, especially on slower links.

Limit Concurrent Migration Sessions

Due to limited upload bandwidth, I also recommend running only one migration at a time:

Set-MigrationEndpoint <EndpointName> -MaxConcurrentMigrations 1 -MaxConcurrentIncrementalSyncs 1

This ensures stable transfers and minimizes the risk of concurrent session timeouts.

Set-MigrationEndpoint (ExchangePowerShell) | Microsoft Learn

Verify Latest Exchange Cumulative Update

Some timeout handling improvements and MRSProxy fixes were introduced in later Exchange 2016 Cumulative Updates. Please confirm the server is updated to the latest CU available from Microsoft Learn

After that, you can apply the configuration changes, restart the IIS and MRS services, and retry a single mailbox migration while monitoring its progress using:

Get-MoveRequestStatistics -Identity <user@domain> -IncludeReport

If the migration completes successfully, proceed with the remaining mailboxes in small batches.

I hope these insights are helpful. Please feel free to reach out if you have any questions or concerns.

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Thank you for your help so far.

I have done the Validate EWS and MRSProxy Connectivity section, no issues there.

I changed the web.config file, this configuration option is new for me. Sadly it didnt changed anything, the timeout remains the same:

The call to https://"url of the server"/EWS/mrsproxy.svc timed out. --> The request channel timed out while waiting for a reply after 00:00:00. Increase the timeout value passed to the call
to Request or increase the SendTimeout value on the Binding. The time allotted to this operation may have been a portion of a longer timeout. --> The request operation did not complete
within the allotted timeout of 00:00:50. The time allotted to this operation may have been a portion of a longer timeout. --> The request channel timed out while waiting for a reply after
00:00:00. Increase the timeout value passed to the call to Request or increase the SendTimeout value on the Binding. The time allotted to this operation may have been a portion of a longer
timeout. --> The request operation did not complete within the allotted timeout of 00:00:50. The time allotted to this operation may have been a portion of a longer timeout.

The Exchange Server 2016 is up to date, i checked it with healthchecker.ps1.

Over the night i was able to move some mailbox with following procedure:

1. Move the user in AD to not Entra synced OU
2. Start-AdSyncSyncCicle
3. Delete softdeleted users permanently in entra
4. Delete softdeleted mailboxes in exchange online:

       get-mailuser "username in exchange online" -SoftDeletedMailUser | Remove-MailUser -PermanentlyDelete
   

5. Move the user in AD to synced OU
6. Start-AdSyncSyncCicle
7. Export and delete all big elements from mailbox to .pst file to shrink the mailbox to minimum (a few MB left)
8. Delete dumpster from mailbox (deleted elements)

      Search-Mailbox -Identity "alias in exchange onprem" -SearchDumpsterOnly -DeleteContent
   

9. Start migration again
10. Completing migration
11. Import exported elements to mailbox again

Now i am stucked with one mailbox again, maybe because in office times the internet connection is overloaded. Will try it in the evening again.

P.S. the personal information was anonymized by me before posting, "domain.tld" is not a valid domain ;-)

Hello Hani-Ng

Yesterday I was able to migrate another 2 mailboxes. Sadly I get the errors again for another 2 Mailboxes.

I am thinking now of backup, delete and recreate the mailboxes in the cloud. Then i could restore the data.

I haven't much informations in the boxes, maybe its the best way to do it.