Mailboxes Stalling in Initial Sync on Hybrid Migration

Question

I am handling our hybrid setup, I'm hitting a wall with mailbox migrations from Exchange 2019 (CU14) to Exchange Online.

Batches start okay via EAC, but 20-30% of mailboxes hang in "Initial Sync" (stuck at 0% or low progress for hours), showing vague "TransientError" or nothing in reports.

Retries don't budge them, while others finish fine. Permissions look solid, no obvious corruption, and it's killing our timeline and need to cut over soon without data risks.

Any help

Answer 1

Hi @Santiago Cruz

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

As your description, 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 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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Answer 2

Today, 28-01-2026, I'm having the same problem.

I'm trying to migrate mailboxes from on-premises Exchange to Microsoft Exchange, and I'm not succeeding. The process gets stuck at 0%, and it's been like this for hours without returning any error message.

Yesterday (and in the days before) I performed several migrations without any problems.