Exchange Migration Issue

Hi @JayJ

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

We understand you’re experiencing an issue where several mailboxes stall during an Exchange migration from Exchange 2016 to Exchange Server SE. The migration batches remain stuck in the Initial Sync phase despite retries, and permissions/configuration appear correct. 

This issue might relate to Workload Management (WLM) throttling. Starting with Exchange 2019, Microsoft introduced Workload Management (WLM) throttling, replacing the old MRS throttling mechanism. By default, WLM limits concurrent mailbox migrations to 10 per server. For example: 

If you create a batch of 50 mailboxes, the first 10 will migrate normally. 

The remaining 40 will stay stalled (such as StalledDueToTarget_MdbReplication or StalledDueToTarget_MdbAvailability) and will not resume, even after retries or service restarts. 

You could try to follow these steps to troubleshoot it: 

Increase the concurrency limit using a Setting Override on the target Exchange server: 

Run this command:

$limit = 40 
New-SettingOverride -Name "Migration-Concurrency-Increase" ` 
  -Component WorkloadManagement ` 
  -Section MdbReplication,MdbAvailability,DiskLatency,CiAgeOfLastNotification ` 
  -Parameters @("MaxConcurrency=$limit") ` 
  -Reason "Increase migration speed" 

To apply the override: 

Restart-Service MSExchangeDagMgmt 

Then, resume the migration batch: 

Resume-MigrationBatch "YourBatchName" 

This might help stalled mailboxes should start migrating again. 

You can refer via:  

Mailboxes are stalled during a migration - Exchange | Microsoft Learn 

If some mailboxes still encounter this issue, you could try to check for corrupted items. 

Get-MoveRequestStatistics -Identity ******@domain.com | FL BadItemsEncountered,Message 

If BadItemsEncountered > 0, increase the limits: 

Set-MoveRequest -Identity ******@domain.com -BadItemLimit 50 -LargeItemLimit 50 –AcceptLargeDataLoss 

Additionally, you could also consider to optimize Migration strategy to break large batches into smaller ones. Schedule large mailbox migrations during off-peak hours to reduce server load. 

I hope this helps. 

Please understand that our initial reply may not always immediately resolve the issue. However, with your help and more detailed information, we can work together to find a solution. 

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