9 min readHow to Migrate Archive Mailboxes Between Microsoft 365 Tenants  

Migrating In-Place Archive mailboxes between Microsoft 365 tenants is one of the most overlooked – and most challenging – parts of a tenant-to-tenant migration. Unlike primary mailboxes, archive mailboxes require separate handling, manual enablement on the target side, and careful attention to licensing and retention policies. 

This guide covers everything you need to know about archive mailbox migration, including prerequisites, licensing, step-by-step instructions, and post-migration tasks to ensure zero data loss. 

What Is an In-Place Archive Mailbox? 

In-Place Archive (also called Online Archive) is an Exchange Online feature that provides users with a secondary mailbox for storing older or less frequently accessed emails. It sits alongside the primary mailbox within the same tenant and helps organizations manage mailbox size while meeting compliance and retention requirements. 

Key characteristics: 

• Operates as a separate mailbox from the user’s primary mailbox 

• Default archive size is 50 GB (Exchange Online Plan 1) or 100 GB (Exchange Online Plan 2) 

• With auto-expanding archiving enabled, the archive can grow up to 1.5 TB through auxiliary archive shards 

• Managed through the Exchange Admin Center (EAC) or PowerShell 

Why Archive Migration Needs Special Attention 

Most native Microsoft cross-tenant migration tools focus on primary mailboxes. Archive mailboxes are not automatically migrated in many scenarios, and several third-party tenant to tenant migration tools also skip them unless explicitly configured.  

Microsoft’s native cross-tenant migration does support auto-expanded archives with up to 12 auxiliary archive mailboxes, but the setup is complex and requires careful attribute matching – including the ArchiveGuid. 

If your users have data in their online archive, you must plan for it separately – or risk losing months or years of archived emails. 

Common challenges include: 

• Archive mailboxes growing into hundreds of gigabytes, requiring auto-expanding archive support on the target 

• Retention policies not carrying over automatically to the new tenant 

• The target archive must be manually enabled before or after migration – it is not created automatically 

• The ArchiveGuid changes during cross-tenant migration, which can affect scripts referencing the old GUID 

Licensing Requirements for Archive Mailbox Migration 

Before you begin migrating archive mailboxes, every target user must hold an Exchange Online archiving license that supports In-Place Archiving. Without the right license, you cannot enable the archive mailbox on the target tenant. 

License	Archive Included	Auto-Expanding (up to 1.5 TB)
Exchange Online Plan 1	No (requires add-on)	No (requires add-on)
Exchange Online Plan 2	✅ Yes	✅ Yes
Exchange Online Archiving add-on	✅ Yes (for Plan 1, Kiosk, E1, F3)	✅ Yes
Microsoft 365 E3 / E5	✅ Yes	✅ Yes
Microsoft 365 Business Premium	✅ Yes	✅ Yes
Office 365 E3 / E5	✅ Yes	✅ Yes

Pre-Migration Checklist 

Before initiating the archive mailbox migration, complete these steps: 

1. Audit source archives - Identify all users with In-Place Archive enabled and note the size of each archive. Use PowerShell: 

Get-Mailbox -Archive -ResultSize Unlimited | Get-MailboxStatistics -Archive 

Also capture the ArchiveGuid for each user: 

Get-Mailbox -Archive -ResultSize Unlimited | Select-Object DisplayName, PrimarySmtpAddress, ArchiveGuid

2. Assign proper licenses on the target tenant - Ensure each target user has Exchange Online Plan 2 or an equivalent license that includes archiving. 

3. Enable auto-expanding archiving on the target - If any source archive exceeds 100 GB: 

Set-OrganizationConfig -AutoExpandingArchive 

4. Manually enable the archive mailbox on the target - Navigate to EAC → Recipients → Mailboxes, select the user, go to Others → Manage mailbox archive, and click Enable. This step is critical – the target archive will not be created automatically. 

5. Document existing retention policies - Export retention tags and MRM retention policies from the source tenant, as these do not migrate automatically. Use Get-RetentionPolicy and Get-RetentionPolicyTag to export the configuration. 

6. Check for holds - Verify whether any source archive mailboxes are under Litigation Hold or eDiscovery hold, as this affects migration behavior. 

Step-by-Step: Migrating Archive Mailboxes with Apps4.Pro 

Apps4.Pro Migration Manager supports seamless In-Place Archive mailbox migration between Microsoft 365 tenants, preserving archived emails, folder structures, and metadata. 

Step 1: Connect Source and Target Tenants 

Register both your source and target Microsoft 365 tenants in the Apps4.Pro Migration Manager. Provide global admin or delegated admin credentials with appropriate Exchange permissions. 

Step 2: Enable Archive on Target Mailboxes 

The In-Place Archive must be manually enabled on each target mailbox via the Exchange Admin Center before migration. For bulk enablement, select multiple mailboxes in the EAC, click More options, then Archive → Enable. Alternatively, use PowerShell: 

Enable-Mailbox -Identity user@domain.com -Archive 

Step 3: Map Source and Target Mailboxes 

Map each source archive mailbox to the corresponding target mailbox. Apps4.Pro supports CSV-based bulk mapping for large migrations. 

Step 4: Configure and Start Migration 

Select the archive mailbox migration option within Apps4.Pro. You can filter by date range to migrate specific time periods or run a full migration. Apps4.Pro handles auto-expanding archive migration of all sizes, including those with multiple auxiliary archives 

Step 5: Monitor and Validate 

Monitor the migration progress in the Apps4.Pro dashboard. Once complete, verify data integrity by comparing item counts and folder structures between source and target archives. 

How Long Does Archive Mailbox Migration Take? 

Migration duration depends on archive size, item count and distribution, network bandwidth, and Exchange Online throttling limits. 

Typical migration performance with Apps4.Pro: 

• Per-mailbox speed: ~1 GB/hour per mailbox 

• Daily throughput: ~20 GB per day per mailbox 

• 50 GB archive: ~2–3 days 

• 100 GB archive: ~5–6 days 

• 500 GB archive: ~25–30 days 

• Archives over 100 GB with auto-expanding: Require additional time because Exchange Online provisions auxiliary storage in 100 GB increments, which can take up to 30 days each 

To optimize throughput: 

• Run migrations during off-peak hours to minimize throttling 

• Use incremental synchronization after the initial pass to capture delta changes 

• Submit large archive migrations well in advance of your cutover date 

• Run parallel migrations across multiple mailboxes simultaneously to maximize overall throughput 

Post-Migration: Configuring Retention Policies in Microsoft Purview 

Retention policies do not migrate between tenants. You must manually recreate them in the target tenant using Microsoft Purview. 

Recreating MRM Retention Policies 

1. Sign in to the Microsoft Purview portal > Solutions > Data Lifecycle Management > Exchange (legacy) > MRM Retention tags. 

2. Recreate each retention tag from the source tenant – specify the retention period, action (e.g., “Move item to archive”), and scope (e.g., “Automatically to entire mailbox”). 

3. Navigate to MRM Retention policies and create a new policy, linking the retention tags you just created. 

4. Assign the new retention policy to target mailboxes via EAC > Recipients > Mailboxes > Mailbox properties > Mailbox policies. 

Force Retention Processing with Managed Folder Assistant 

After assigning retention policies, the Managed Folder Assistant (MFA) processes mailboxes approximately once every 7 days by default. To apply retention policies immediately: 

Start-ManagedFolderAssistant -Identity user@domain.com 

This triggers the MFA instantly, so retention tags and archiving actions apply right away without waiting for the next processing cycle. 

Retention Policy for Archive Content 

Create a separate retention policy for archive content to prevent unintended deletion. When a single retention policy is applied to the primary mailbox, it also applies to the archive – so use distinct policies if your retention requirements differ between the two. 

Alternative Approach: eDiscovery and Compliance Search 

Some administrators use Compliance Search and eDiscovery as an alternative method to export archive mailbox content – particularly when third-party tools are not available. You can run a Content Search in the Microsoft Purview compliance portal scoped to the source archive mailbox, export the results as a PST, and then import the PST into the target archive using New-MailboxImportRequest. However, this approach is significantly slower and does not preserve folder structures as cleanly as direct migration tools. 

Common Pitfalls and Best Practices 

• Don’t forget to enable the archive first. The most common migration failure occurs when admins attempt to migrate archive data to a target mailbox that doesn’t have an archive enabled. 

• Allow time for auto-expanding archive provisioning. After enabling auto-expanding archiving, Exchange may take up to 30 days to provision each additional 100 GB of auxiliary archive storage. 

• Run incremental migrations. For large archives, perform an initial bulk migration followed by incremental passes to capture any changes made during the migration window. 

• Verify licensing before migration day. An unlicensed target mailbox cannot have an archive enabled, which will block the entire archive migration for that user. 

• Test with a pilot group. Migrate a small batch of archive mailboxes first to validate your process, timing estimates, and data integrity checks. 

• Account for ArchiveGuid changes. The archive mailbox GUID changes during cross-tenant migration – update any scripts or automation that reference the old ArchiveGuid after migration. 

• Run Managed Folder Assistant post-migration. After applying retention policies, force processing with Start-ManagedFolderAssistant to ensure policies take effect immediately. 

Can you migrate archive mailboxes between Microsoft 365 tenants? 
Yes, but the target archive must be manually enabled via EAC before migration. Tools like Apps4.Pro Migration Manager handle the data transfer while preserving folder structures and metadata. 

Frequently Asked Questions