Microsoft 365 Tenant-to-Tenant Migration Guide: How to Move Mailboxes During an M&A or Rebrand

A merger is finalized, a rebrand is announced, or a new division is launched, and now two Microsoft 365 environments need to be consolidated. More often than not, such projects mean that a Microsoft 365 tenant-to-tenant migration will be initiated and placed on the roadmap for the next fiscal year.

Mailboxes, calendars, and contacts need to be transferred to the new tenant in a way that minimizes disruptions to the flow of communication and avoids any interruptions or delays to the user experience. In this article, we will examine the built-in capabilities offered by Microsoft for cross-tenant migration, identify potential roadblocks, and single out situations when a dedicated migration tool can provide benefits.

What Tenant-to-Tenant Migration Actually Involves

An Office 365 tenant migration is when a mailbox with its identity has to be moved out of one Microsoft 365 tenant and into another, which only somewhat resembles the nature of an on-premises Exchange server migrated in the cloud. The actual transfer of the mailbox content is handled by a service from Microsoft called Mailbox Replication Service (MRS), which we use for hybrid and cutover migrations too; it’s simply pointed at another tenant instead of an on-premises server. The unfortunate reality about any cross-tenant Microsoft 365 mailbox migration is that identity has to be wrestled elsewhere — off the original tenant, in fact. This is why most of these projects take longer and are not as simple (on premise) domain migrations. You need an existing matching object in the target tenant before any mail can move for each migrating user.

Prerequisites Before You Start

The key prerequisites for any cross-tenant mailbox discovery are:

• Administrative rights in both tenants that can create app registrations, an organization relationship, and a migration endpoint (in practice, Global Administrator access on each side).
• For each migrating user, both source and target will need a license for Exchange Online and Teams.
• A mail-enabled security group in the source to control which mailboxes are to migrate.
• MailUser objects pre-created in the target tenant, with the source mailbox’s ExchangeGuid stamped onto them before any license is assigned.

Migrating Mailboxes with Microsoft’s Native Cross-Tenant Feature

1. Register the Migration Application: Register the migration application in Microsoft Entra ID on the destination tenant and obtain a client secret for it.
2. Configure the Organization Relationship: Configure the organization relationship on the target tenant, with MailboxMoveEnabled set to true so that it references the source tenant.
3. Create the Migration Endpoint: Create an Exchange Online migration endpoint using the app’s client ID and secret to re-establish the connection to the source.
4. Complete Identity Mapping: Complete the identity mapping by writing mailbox attributes from the source objects onto the target MailUsers, and assign Exchange and Teams licenses only after that completes.
5. Create and Start the Migration Batch: Scope and launch the batch. Execute the New Migration Batch available from the Move Mailboxes management role, which is targeted against the mail-enabled security group created earlier.
6. Monitor and Complete the Migration: Monitor and complete the move. Once done, the source mailbox turns into a MailUser, and its targetAddress is stamped for continued mail routing between the two tenants.

This only transfers mail, contacts, calendar items, tasks, notes, and items from the user’s Recoverable Items folders, and nothing beyond this set.

What the Native Feature Doesn’t Cover

Public folders, Microsoft 365 Group mailboxes, and Teams chat data stored inside Exchange mailboxes are excluded from the move. Distribution lists and external contacts aren’t part of mailbox data migration and, like public folders, must be recreated in the target tenant ahead of time or mail routing will fail for those who rely on them. Mailboxes under any type of hold are blocked until the hold is released. This migration only affects Exchange Online; SharePoint sites and OneDrive accounts require a separate cross-tenant migration path and, despite a single per-user license covering both, it’s another project to work on in parallel with a mailbox migration for a few hundred users.

When a Professional Migration Tool Makes Sense

In a complete Microsoft 365 rebrand or M&A email migration involving all classes of resources in the old brand’s tenant that must be moved to the surviving tenant, mailboxes, SharePoint sites, and OneDrive accounts are among the workloads that need to be consolidated. This is where a tool such as DRS Softech Office 365 Tenant to Tenant Migration Tool comes in useful, as this tool consolidates all three workloads in a single console.

The tool uses the Microsoft Graph API, Modern Authentication, and supports MFA when it connects to MS 365. It supports one-to-one, one-to-many, many-to-one, and many-to-many mapping types for these workloads; it uses incremental migration so as not to reprocess already migrated data (incremental switch); hash-based deduplication of emails, SharePoint files & OneDrive content. It also provides detailed reports for each migration — which items were transferred, skipped, or failed and would help significantly when compliance auditors require proof of what was migrated.

Native Migration vs. a Professional Tool

Capability	Native Cross-Tenant Migration	DRS Softech Tenant Migration Tool
Workload covered	Exchange Online mailbox only	Exchange Online, SharePoint Online, OneDrive
Interface	PowerShell and the Exchange admin center	Graphical dashboard
Repeat session	Manual batch re-run	Built-in skip-previously-migrated
Mapping options	One MailUser per migrating mailbox	One-to-one, one-to-many, many-to-one, many-to-many
Duplication	Not include	Built-in email, SharePoint, and OneDrive deduplication
Reporting	Get-MigrationBatch status only	Detailed per-task migration reports

Conclusion

Whatever path the project takes, migration of mailboxes between tenants comes down to the same discipline: map all the mailboxes before touching them, know precisely what Microsoft’s platform will and will not carry across on its own, and verify that against a report instead of a guess. Native cross-tenant mailbox migration is a good fit for straight transfers of mailboxes only. When SharePoint, OneDrive, deduplication, or flexible mapping come into play, DRS Softech Microsoft 365 Tenant-to-Tenant Migration Tool will save more hours than it costs to set up.

Frequently Asked Questions

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