Migrating email archive over 100GB but below 240GB

Question

Hi all,

If a user has a mailbox of less than 100Gb, but an archive mailbox of lets say 150GB with auto expanding on, will the emails be migrated and the archive mailbox be adjusted instantly, or will the migration wait until the auto expand has added the space before migrating the user.

Here are three scenarios i'd like clarity on:

1. (Primary mailbox under 100GB, Archive mailbox under 90GB) is migrated.
2. (Primary mailbox under 100GB, Archive mailbox between 90 and 100GB) is migrated.
3. (Primary mailbox under 100GB, Archive mailbox greater than 100GB) is migrated.

 Thanks

Answer 1

Hi @Raymond Brooks

Thank you for reaching out with your questions about mailbox migration, especially regarding archive mailboxes with auto-expansion enabled. Understanding this process is key to a smooth transition, and I'm happy to clarify how it works for the scenarios you've presented. 

When migrating mailboxes, the system doesn't wait for auto-expansion to kick in before starting the migration. Instead, the migration begins as soon as it's scheduled. If an archive mailbox exceeds its storage limit during the process, the auto-expansion feature will automatically provide the extra space needed. This happens on the fly, so there's no interruption to the migration. However, do keep in mind that it may take anywhere from a few days to up to 30 days for the expansion to be fully completed, depending on the volume of data and infrastructure. 

Now, here's how this works with your specific scenarios: 

Scenario 1: Primary mailbox under 100GB, Archive mailbox under 90GB  

In this situation, both your primary and archive mailboxes are well within their initial limits. The migration will proceed smoothly, and there won’t be a need for auto-expansion on the archive mailbox. 

Scenario 2: Primary mailbox under 100GB, Archive mailbox between 90GB and 100GB 

Similar to the first scenario, both mailboxes are within their initial limits. The migration will occur, and the existing data will be moved. While the archive is close to 100GB, auto-expansion won't be triggered unless the migrated data causes it to exceed 100GB after the transfer. 

Scenario 3: Primary mailbox under 100GB, Archive mailbox greater than 100GB (e.g., 150GB with auto-expanding on) 

Here's where auto-expansion really comes in handy. When migration begins, the system starts transferring the 150GB archive data. As the archive exceeds 100GB, auto-expansion kicks in to automatically add the necessary storage, ensuring a seamless migration process. 

It’s important to note that auto-expanding archiving is supported only for individual or shared mailboxes with a growth rate of up to 1GB per day. If the archive mailbox grows faster than this or you run into any issues, we strongly recommend contacting Microsoft Support. They can assist in temporarily increasing storage to speed up the process, allowing auto-expansion to take over smoothly. 

In short, the migration copies the data at the start, and auto-expanding archives grow on demand. There's no need to trim or pre-provision space—Exchange handles it for you. 

For more detailed technical information on auto-expanding archiving and large mailbox migrations, you can refer to Microsoft's official documentation: 

Learn about auto-expanding archiving: https://learn.microsoft.com/en-us/purview/autoexpanding-archiving?view=o365-worldwide 

Enable and configure auto-expanding archiving in Exchange Online: https://learn.microsoft.com/en-us/purview/enable-autoexpanding-archiving 

Migrate large on-premises mailboxes to Microsoft 365: https://learn.microsoft.com/en-us/exchange/mailbox-migration/large-mailbox-migration-from-onpremises 

I hope this information clarifies the process and helps you prepare for a smooth migration. If you have any further questions or need additional assistance, please don't hesitate to reach out. 

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