Large Mailbox Migration to Exchange Online

Migrating large mailboxes is challenging for enterprise Exchange teams, especially when mailboxes are over 100 GB or contain extensive recoverable items. Using Exchange Messaging Records Management (MRM) to reduce mailbox size before migration can speed up moves to Exchange Online.

Why Use MRM Before a Large Mailbox Migration?

Many organizations place mailboxes on litigation hold or in-place hold, causing the recoverable items in these mailboxes to grow significantly, often exceeding the 100 GB quota in Exchange Online. Quota adjustments can be requested, allowing up to about 240 GB for the combined size of the primary mailbox and recoverable items. Still, it's common for recoverable items alone to surpass this limit.

MRM lets you move content from the primary mailbox to an archive mailbox, reducing the primary's overall size. The archive mailbox may be hosted on-premises or in Exchange Online. Setting up the archive in Exchange Online is usually simpler, reducing the need for additional mailbox migrations. Occasionally, this process can result in the archive mailbox's recoverable items exceeding the 240 GB cap. Therefore, creating the archive in Exchange Online remains the most efficient solution.

Prerequisites

• Archive mailbox created in Exchange Online
• The archive mailbox must have the correct routing domain configured as the ArchiveDomain value
• OAuth enabled in Exchange
• AutoExpandingArchiveEnabled must be enabled for either mailbox or entire organization

MRM Configuration

The required retention policy tag is dependent upon where the data is located within the mailbox. Our primary focus is on recoverable items for mailboxes on holds; therefore, we need to create a tag to move recoverable items older than x number of days to archive.

New-RetentionPolicyTag -Name RecoverableItems_31_MoveToArchive -MessageClass * -RetentionAction MoveToArchive -AgeLimitForRetention 31.0:0:0 -Type RecoverableItems -RetentionEnabled:$True -Comment "Archive all items from the Recoverable Items over 31 days"

This tag must be added to a retention policy, and the retention policy must be assigned to the user being migrated. Once this is done, you can start the managed folder assistant (MFA) to move items into the remote archive.

Start-ManagedFolderAssistant user@contoso.com

Note: A new retention policy may need to be created specifically for these larger mailboxes.

Speed up expanded archives

One issue with migrating large mailboxes is the delay caused by auto-expanding archives. Thankfully, this delay depends on Exchange processes, which we can observe and activate manually when needed.

The first thing to do is keep an eye on your archive mailbox size. Once it hits 90GB, auto-expansion should kick in. To track this, check the mailbox statistics for the archive mailbox.

Get-MailboxStatistics  <guid of MainArchive shard of MailUser> | fl *itemCount,*ItemSize

The results indicate that the TotalDeletedSize has reached 100GB, which is the established quota limit. At this threshold, the auxiliary archive should trigger the next time the managed folder assistant (MFA) runs against the mailbox. Manually start the MFA to expedite this process:

Start-ManagedFolderAssistant <guid of MainArchive shard of MailUser>

Confirm MFA has completed by checking the ELCLastSuccessTimestamp:

(Export-MailboxDiagnosticLogs -Identity <guid of MainArchive shard of MailUser> -ExtendedProperties).mailboxlog | Select-Xml -XPath "//MailboxTable/*" | select -ExpandProperty Node | ? {$_.name -like "ELC*"}

Once the auxiliary archive becomes available, Exchange will initiate the process of copying data into the new mailbox. The MFA must be triggered again to start copying data. Then we can proceed to verify whether any folders have been ghosted using the following steps:

$folders = Get-MailboxFolderStatistics -FolderScope recoverableitems  <guid of MainArchive shard of MailUser>

$folders | ?{-Not $_.ContentFolder -and $_.VisibleItemsInFolder} | Sort-Object LastMovedTimeStamp | ft FolderSize,LastMoved*,Content*

These folders have been copied to an auxiliary archive but are not yet expired on the MainArchive, leaving about 43GB of storage pending release. MFA will free this space after its next run, once five days have passed since "11/28/2024 10:25:07 PM".  

Our monitoring speeds up the process since MFA may take several days to finish. After five days from the LastMovedTimeStamp, we manually start the MFA using the following command:

Start-ManagedFolderAssistant <guid of MainArchive shard of MailUser>

You will notice these folders shrinking and the primary archive gaining free space.

If there are no ghosted folders and the mailbox is full or exceeds 90GB of recoverable items, start MFA to trigger expansion. It may help to run MFA more than once and confirm that it completed successfully.

Conclusion

Using Messaging Records Management (MRM) ahead of a large mailbox migration helps reduce primary mailbox and recoverable items pressure by moving older content into the archive, improving the likelihood of staying within Exchange Online limits and accelerating move performance. With the right prerequisites in place, you can actively monitor archive growth and expansion. When the archive approaches capacity or when ghosted folders are older than five days, targeted monitoring and triggering MFA against a mailbox can accelerate expansion and free space sooner—keeping migrations on track.

• Use MRM to move Recoverable Items older than your chosen threshold into the archive before starting migrations.
• Track archive statistics (especially TotalDeletedItemSize/TotalDeletedSize) to anticipate auto-expansion and identify bottlenecks.
• Monitor ghosted folders and run MFA after the relevant LastMovedTimeStamp interval to accelerate cleanup.