How to Migrate Shared Mailboxes Between Office 365 Tenants?

Migrating shared mailboxes between Office 365 tenants can be a challenging task, especially for organizations going through mergers, acquisitions, or restructuring. Unlike user mailboxes, shared mailboxes don’t require a license (up to 50 GB), but they still contain critical business data that must be moved securely. In this guide, we’ll walk through the process of migrating shared mailboxes between Office 365 tenants, the challenges involved, and the best practices to ensure a smooth transition.

Why Do You Need to Migrate Shared Mailboxes?

There are several reasons organizations may need to move shared mailboxes across tenants:

Mergers & Acquisitions: When two companies combine, their mail systems often need consolidation.

Business Restructuring: Departments or units moving to different environments.

Domain Consolidation: To unify collaboration tools under a single Microsoft 365 tenant.

Compliance & Data Retention: Ensuring historical communications remain accessible.

Challenges in Migrating Shared Mailboxes

Migrating shared mailboxes isn’t as straightforward as moving user mailboxes. Some challenges include:

1. No Native Microsoft Tool: Microsoft doesn’t provide a direct built-in tool for tenant-to-tenant shared mailbox migrations.

2. Permissions Transfer: Shared mailboxes often have multiple users with permissions, which must be migrated carefully.

3. Downtime Risks: Business-critical communication may be disrupted if migration isn’t planned properly.

4. Data Integrity: Maintaining folder structures, metadata, and shared permissions is crucial.

Pre-Migration Checklist

Before starting the migration, ensure the following steps are taken:

Inventory Shared Mailboxes: List all shared mailboxes, sizes, and permissions.

Plan Migration Strategy: Decide if you’ll use a third-party tool or manual methods.

Communicate with Users: Inform users about possible downtime and changes in access.

Backup Data: Always have a backup in case of migration failure.

Prepare Target Tenant: Ensure target tenant is set up with required shared mailboxes.

Methods to Migrate Shared Mailboxes

1. Manual PST Export/Import Method

This method is suitable for small mailboxes and non-critical data.

Export shared mailbox data to a PST file using Outlook or eDiscovery.

Create a new shared mailbox in the target tenant.

Import the PST file into the new mailbox.

Reassign permissions to users.

Pros: Simple, free method.

Cons: Time-consuming, risk of data loss, permissions not migrated automatically.

2. Using Microsoft’s Cross-Tenant Mailbox Migration (Limited Support)

Microsoft is gradually rolling out cross-tenant mailbox migration for user mailboxes, but support for shared mailboxes is limited.

If eligible, you can set up cross-tenant migration via PowerShell and admin centers.

Not widely available and often not recommended for large-scale shared mailbox moves.

3. Third-Party Migration Tools (Recommended)

Several enterprise-grade tools simplify tenant-to-tenant shared mailbox migration:

Quest On Demand Migration

BitTitan MigrationWiz

CodeTwo Office 365 Migration

Kernel Office 365 Migration

Steps with Third-Party Tools:

Connect both source and target tenants.

Select shared mailboxes to migrate.

Map permissions and users.

Start migration with minimal downtime.

Pros: Fast, secure, automates permissions, supports large-scale migration.

Cons: Paid solutions, licensing required.

Best Practices for a Smooth Migration

Migrate in Batches: Move smaller groups of mailboxes first to test migration performance.

Set Proper Permissions: Reapply mailbox permissions in the target tenant.

Minimize Downtime: Schedule migrations during off-peak hours.

Validate Data: Verify mailbox items, calendar entries, and folder structures after migration.

Decommission Old Mailboxes: Once verified, remove old mailboxes to avoid confusion.

Final Thoughts

Migrating shared mailboxes between Office 365 tenants requires careful planning and execution. While manual methods work for smaller organizations, businesses with larger environments should invest in third-party migration tools to ensure a secure, efficient, and disruption-free process.