Migration Guidance

Question

Hi All,

I am planning to migrate mailboxes from Google Workspace to Microsoft 365 using the Microsoft FastTrack migration service. I previously performed a migration about a year ago but encountered issues related to mail forwarding.

I have now created a new FastTrack project and completed the prerequisites. Under FastTrack → Migrations → Manage Events, I am adding a new event and have selected Event Type: Synchronization.

My understanding is that the Synchronization event performs the initial mailbox sync from Google Workspace to Microsoft 365. After that completes, I need to create another event with Event Type: Completion.

I would like clarification on the following points:

1. What is the exact difference between the Synchronization and Completion event types?
   • Is it correct to assume that Synchronization migrates the majority of mailbox data (for example, ~95%), and the Completion event migrates the remaining delta changes before final cutover?
   At what stage should mail forwarding be configured on the Google mailboxes to route new emails to Microsoft 365? Should forwarding be enabled after the Synchronization event?

     Or should it only be enabled after the Completion event?
   
     If forwarding is configured immediately after the Synchronization event, could this result in duplicate emails?
   
     If forwarding is configured only after the Completion event, is there any risk of users missing emails during the transition?
   

Additionally, during my previous migration in Microsoft 365, I noticed that the migration batch status showed “Needs Approval.” When I clicked Approve migration batch, it prompted me to confirm the approval.

Does the “Needs Approval” status indicate data consistency issues?

Or does it mean that some emails could not be synced and require administrative confirmation before completing the migration?

I would appreciate guidance on the recommended sequence of steps to avoid duplicate or missing emails during the cutover

Answer 1

Hi @Glenn Maxwell

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

Regarding to your concerns: 

Differences between Synchronization and Completion event types 

You are correct about how Synchronization works. 

During a Synchronization event, Exchange Online (EXO) migrates approximately 95% of each mailbox in the batch. After this phase completes, EXO will continue to periodically sync changes to maintain that 95% state until you run a Completion event. 

When to enable mail forwarding in Google Workspace 

Based on my research, you should NOT enable forwarding immediately after Synchronization, because: Synchronization is not the final stage of the migration workflow. If forwarding is enabled too early, new emails arriving in Google will forward to M365, but FastTrack will still be syncing, this may cause duplicate emails.

You could enable it after the Completion event, together with updating the MX records. The migration tool performs one last delta sync and then stops. At that moment, the system automatically enables Google > M365 forwarding and disables the reverse direction.

Since no more synchronization occurs after Completion, mail flows only one way, ensuring no duplication. Afterward, admins update the MX records so new mail flows directly into M365, while forwarding from Google acts as a temporary fallback for any straggling messages.

For the “Needs Approval” status 

This status is not critical, nor does it indicate a data inconsistency. This might relate to the migration engine encountered some items that cannot be migrated (such as, corrupted emails, size-limit violations, unsupported formats). These items require an admin to approve skipping them so that the migration can continue with all remaining items. 

You can approve the skipped items using PowerShell: 

# Approve skipped items for the entire batch 
Set-MigrationBatch -Identity "BatchName" -ApproveSkippedItems 
# Or approve skipped items for an individual user 
Set-MigrationUser -Identity "******@domain.com" -ApproveSkippedItems 

After approval, the migration will continue. Any skipped items are logged in the migration report for later review if needed. 

You can refer more via:  

Data Migration - FastTrack – Microsoft 365 | Microsoft Learn 

FastTrack Migrations Learning Center 

Gmail migration tool - stuck at NeedsApproval - Microsoft Q&A 

Complete mailbox migration with bad items - ALI TAJRAN 

Note: Microsoft is providing this information as a convenience to you. These sites are not controlled by Microsoft, and Microsoft cannot make any representations regarding the quality, safety, or suitability of any software or information found there. Please ensure that you fully understand the risks before using any suggestions from the above link. 

I hope this helps. 

Please feel free to correct me if I misunderstood your request. If you have any additional concerns, feel free to comment below. I would be more than happy to assist. 

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