I wanted to bring a peculiar issue to your attention that I encountered while performing a large mailbox migration from Google Workspace to Microsoft 365. As you know, meticulous planning is crucial for these migrations, and I ensured that everything was set up perfectly. I designed my plan carefully, created an XML file, and enabled the Set-Mailbox -ELCProcessingDisabled:$true for each user’s mailbox. Given that I was dealing with a particularly large mailbox of around 200 GB, I split the batches into separate XML and CSV files per mailbox to speed up the synchronization process.

However, I ran into a persistent error with a specific user’s Archive mailbox. The status would remain in provisioning and eventually fail, displaying the error:

Error: MigrationProvisioningPermanentException: MailboxLocations: Cannot parse the following: SubstrateExtension. –> MailboxLocations: Cannot parse the following: SubstrateExtension.

This error indicates an issue with parsing the SubstrateExtension mailbox type during the migration, which can occur when moving data from Google Workspace to Microsoft 365 or during other mailbox provisioning tasks.

Despite following the recommended steps from various forums, I couldn’t resolve the issue. Then, an idea struck me, reminding me of my earlier days with Exchange starting from version 5.5. I decided to return to basics, decoding the error line by line and referring back to the Microsoft article on migrating large mailboxes from Google or other IMAP sources to Microsoft 365 Exchange.

I discovered that the Archive mailbox couldn’t locate the mailbox, leading to the provisioning failure. To address this, I ran the command Get-MailboxLocation -User joannav@contoso.com to find the MailboxGuid. Since the primary mailbox had synchronized correctly, I created a new XML file with only the Archive Mailbox, using the GUID-of-Existing-Mailbox instead of MainArchive. This adjustment allowed the syncing to start without any issues.

I wanted to share this experience with you in case you encounter a similar problem. Sometimes, going back to the basics and understanding the root of the error can provide the solution.

Hope this helps …