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Tenant to Tenant Migration: Moving Between Microsoft 365 Tenants

A tenant to tenant migration moves users, mailboxes, files and domains from one Microsoft 365 tenant to another, most often after a merger, acquisition, divestiture or rebrand. Microsoft’s native cross-tenant mailbox migration moves mailbox content between Exchange Online tenants, and a separate cross-tenant OneDrive migration moves files. Both require a per user Cross-Tenant User Data Migration license, and most projects also need careful planning for identities, domains, Teams and coexistence.

This guide explains what a Microsoft 365 tenant migration involves and how to plan one. It is part of our email migration services; for the wider program, see the Office 365 migration guide and checklist.

When Organizations Need a Tenant to Tenant Migration

  • Mergers and acquisitions that consolidate two companies into one tenant.
  • Divestitures that move a business unit into its own new tenant.
  • Rebrands and consolidation of several regional or legacy tenants.
  • Licensing or compliance changes such as moving to a different cloud or region.

What Moves and What Needs Rework

WorkloadNative optionNotes
Mailboxes: mail, calendar, contacts, tasks, notesCross-tenant mailbox migrationIncludes recoverable items; the source mailbox is removed after the move
OneDrive filesCross-tenant OneDrive migrationSeparate process, covered by the same per user license
Mailbox and calendar permissionsNot carried across tenantsMove delegates and connected users in the same batch and restamp Send on Behalf
Microsoft 365 groups and TeamsNot moved by mailbox migrationRecreate or use partner tooling; Teams meeting links must be recreated
Mailboxes on holdBlockedRelease or plan for holds before moving
DomainsMoved manuallyA domain can only be verified in one tenant at a time

Prerequisites for Cross-Tenant Mailbox Migration

  • Licensing. A one-time per user Cross-Tenant User Data Migration add-on license for every migrated user. Migrations fail without it.
  • Trust between tenants. An application in the target tenant with mailbox migration permission, admin consent from both tenants, a migration endpoint and organization relationships.
  • Scoping. A mail-enabled security group in the source tenant that defines which mailboxes can move.
  • Target identities. Mail users in the target tenant stamped with matching Exchange GUIDs, archive GUIDs and x500 addresses before they are licensed.

How a Tenant to Tenant Migration Works

  1. Discover and scope. Inventory users, mailboxes, archives, OneDrive data, groups, Teams, applications and holds in both tenants.
  2. Design identity and domains. Decide when each domain moves, how users sign in during the transition and how the address book is shared.
  3. Prepare the tenants. Set up the trust, scoping groups, target mail users and licensing.
  4. Pilot. Move a small batch with delegates and shared mailboxes and confirm mail flow, calendars and devices.
  5. Migrate in batches. Move mailboxes and OneDrive together for each group, with users and their delegates in the same batch.
  6. Cut over the domain. Remove the domain from the source, verify it in the target and switch MX and Autodiscover, usually in a planned weekend window.
  7. Clean up. Rebuild permissions, Teams and groups, reconfigure devices, and decommission or archive the source tenant.

Planning Coexistence and the Domain Cutover

Because a domain can only live in one tenant, many projects use a temporary address or subdomain in the target while users move, then cut the primary domain over at the end. During coexistence, shared address books and free/busy keep the two companies working together, and clear communication tells users when their sign-in name and Outlook profile will change.

Common Tenant Migration Mistakes

  • Forgetting the per user migration license until batches fail.
  • Licensing target users too early, which creates empty mailboxes instead of mail users ready to receive the move.
  • Splitting delegates from their managers across batches.
  • Ignoring Teams, groups and SharePoint that mailbox migration does not move.
  • Leaving holds in place that block mailbox moves.

Planning a tenant to tenant migration?

RockTeam plans and delivers Microsoft 365 tenant migrations for mergers, acquisitions and divestitures, including identities, domains, mailboxes, OneDrive and coexistence. RockTeam is an official HCL Business Partner and a long time Microsoft collaboration specialist. Start with a no cost assessment.

See Email Migration Services

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Tenant to Tenant Migration FAQs

Moving users, mailboxes, files and domains from one Microsoft 365 tenant to another, usually after a merger, acquisition, divestiture or rebrand.

Yes. Cross-tenant mailbox migration moves mailbox content between Exchange Online tenants, and cross-tenant OneDrive migration moves files. Both require a per user Cross-Tenant User Data Migration license.

Mailbox and calendar permissions across tenants, Microsoft 365 groups, Teams chat content, mailbox signatures and Outbox items. Mailboxes on hold cannot move until the hold is addressed.

Yes, but a domain can only be verified in one tenant at a time, so the domain is moved in a planned cutover, usually after most users have migrated.

Small tenants can move in weeks. Large mergers usually run in batches over several months, driven by data volume, domain timing and coexistence needs.