A tenant to tenant migration usually arrives with a deadline attached to it. A business has been bought, sold or merged, and somebody has promised that everyone will be on one email system by a certain date. The technical work is well documented. The planning around it is where projects go wrong.
A tenant to tenant migration moves mailboxes, OneDrive files and SharePoint content from one Microsoft 365 tenant into another. Microsoft supports most of it natively, but it needs a paid per-user licence; the two tenants cannot share a domain name; and several things people assume will move do not move at all.
Microsoft’s own guidance is blunt about the licence: migrations fail without it, and there are no exceptions. It is the single most common reason a first attempt stops before it starts.
What Is a Tenant to Tenant Migration?
It is the move of users and their content from one Microsoft 365 tenant into another. A tenant is the separate instance of Microsoft 365 your organisation owns, holding your users, mailboxes, files and settings. When two organisations come together, they usually end up with two of them and a decision to make.
Microsoft provides native tools for most of the work. Mailboxes move with cross-tenant mailbox migration, and files move with cross-tenant OneDrive migration. The content never leaves the Microsoft cloud while it moves, which removes a lot of risk compared with exporting and re-importing data yourself.
When Does This Come Up?
Almost always after a corporate event. A merger or acquisition is the common one. Divestitures create the reverse problem, where part of the business has to leave and take its data with it. Rebrands and group restructures produce the same job without the deal behind them.
The trigger matters because it sets the deadline, and the deadline is usually not negotiable. Transputec runs these as part of modern workplace projects, and the pattern is consistent: the date is fixed months before anyone has looked at what is actually in the two tenants.
Facing a Tenant Migration Deadline?
Transputec has run this exact move. For Incisive Media it migrated 130 user mailboxes and 100 shared mailboxes, along with OneDrive files, Teams channels and SharePoint sites, across a single weekend after a week of preparation. Talk to our team about the planning work behind a tenant to tenant migration.
Get a Strategic ConsultationWhat Actually Moves, and What Does Not?
Less than most people assume, and the gaps are specific. Microsoft lists exactly what a cross-tenant mailbox move carries: email, contacts, calendar, tasks and notes, plus the Recoverable Items folders. Anything outside that list needs a different plan or a third-party tool.
| Moves natively | Does not move |
|---|---|
| Email, contacts, calendar, tasks and notes | Teams chat history |
| Recoverable Items: deletions, versions and purges | Teams meeting links, which break even though the meetings move |
| OneDrive files, leaving a redirect at the old location | Anything sitting in the Outlook Outbox |
| Shared mailboxes, keeping their store permissions | Send On Behalf Of delegation, which has to be reapplied |
Teams is the one that surprises people. A tenant to tenant migration moves the meetings but not their joining links, so they have to be deleted and recreated. Chat history does not come across at all, though a source tenant administrator can still search and export it afterwards.
Do You Need a Special Licence?
Yes, and it is not part of your normal subscription. Microsoft requires a Cross Tenant User Data Migration licence for every user you move. It is a one-time fee per user, it can sit on either the source or the target account, and the same licence covers both the mailbox and the OneDrive move.
It is sold as an add-on to most of the usual plans, including Microsoft 365 Business Basic, Standard and Premium, Microsoft 365 F1, F3, E3 and E5, and the Office 365 equivalents. Buying it is a procurement step with a lead time, not a checkbox, so it belongs at the start of the plan rather than the week before cutover.
Skip it and the migration simply fails with a licence error. Microsoft’s documentation states plainly that it does not offer exceptions. This is the most common reason a Microsoft 365 tenant migration stalls on its first attempt.
What Happens to Email on the Day?
More than users expect, and it needs telling in advance. Only one tenant can own a domain name, so the source and target tenants cannot share one. When a mailbox lands in the new tenant, the old primary email address is not there with it, and Outlook does not know where to look.
The practical consequence is that every user rebuilds their Outlook profile on day one, using their new username and email address, then waits while the mailbox resyncs to their machine. On a large move that is a lot of simultaneous downloading, so it needs planning against your network capacity rather than being left to chance.
Mail flow in the meantime behaves like an Exchange hybrid setup. The old tenant forwards to the new one, and messages pass through the security and transport rules of the source tenant first, then the target. If you have run into Exchange Server end of life at the same time, that is worth mapping carefully, because you are then changing two things at once.
How Long Does a Tenant to Tenant Migration Take?
Weeks of preparation, then a cutover measured in hours. The copying itself happens quietly in the background with no impact on users, which is why Microsoft recommends submitting mailbox batches around two weeks before the cutover date rather than starting on the night.
Microsoft sets working limits that shape the schedule. Keep mailbox batches under 2,000 at a time. Up to 4,000 OneDrive accounts can be queued at once. A OneDrive account tops out at 5 TB or one million items, and the file path cannot exceed 400 characters in total, which catches deeply nested folders.
One limit deserves its own sentence. Cross-tenant moves are a single pass. There is no incremental or delta sync afterwards, so anything a user changes in the old tenant after their content moves is left behind. That is why the sequencing of communications matters as much as the technical batching.
What Catches People Out?
The blockers are rarely the big things. They are small conditions that stop a batch dead, and each one is easier to find in a survey than at two in the morning on cutover weekend. These are the ones worth checking first.
- Legal holds. A OneDrive account under a hold policy is blocked. The hold has to come off, the data moves, then the hold goes back on in the new tenant.
- Existing OneDrive sites in the target. If the user already has one there, the migration fails and cannot overwrite it. Site creation should be restricted in the target tenant beforehand.
- Purview Customer Key encryption. If service encryption with a customer key is enabled on the source, the move fails.
- Apostrophes in usernames. A name like o’brien in a URL fails with an invalid character error, which is a genuinely awkward thing to discover late.
- Government clouds. The native OneDrive feature is not supported for GCC, GCC High or DoD tenants.
Where the native tenant to tenant migration tools stop, third-party tools take over. For the Incisive Media migration Transputec used MigrationWiz to move mailboxes, OneDrive, Teams channels and SharePoint together, which is a common choice when the scope goes past what Microsoft moves on its own.
Your Tenant to Tenant Migration Checklist
Work through these before you agree a date.
- Buy the Cross Tenant User Data Migration licences early. Nothing moves without them and there are no exceptions.
- Confirm the two tenants do not share a domain name, and decide which domain the merged business will use.
- Inventory what is actually in scope: user mailboxes, shared mailboxes, OneDrive, SharePoint sites, Teams and anything authenticating against the old tenant.
- Find the blockers now: legal holds, existing OneDrive sites in the target, customer key encryption, long file paths and awkward usernames.
- Decide what happens to Teams chat, given it does not migrate, and tell people before they lose sight of it.
- Plan day one support. Every user rebuilds an Outlook profile and resyncs, so the service desk needs to be staffed for it.
If the move is part of a wider change of platform, our cloud migration work covers the discovery and sequencing around it.
Conclusion
A tenant to tenant migration is a well-trodden piece of work with a small number of hard edges. Microsoft moves the mail and the files reliably, and it does it without the content leaving its cloud. What it does not do is move Teams chat, fix meeting links, carry delegation, or let you run a second pass to pick up stragglers.
That is why the planning carries the project. The businesses that come through this calmly bought the licences early, found the blocked accounts before the batches ran, and told users exactly what day one would feel like. Transputec has done this at scale and the weekend is usually the quiet part, because the awkward discoveries happened weeks earlier.
FAQs
How Long Does a Tenant to Tenant Migration Take?
Typically a few weeks of preparation and a cutover over a weekend. The copying runs in the background without affecting users, and Microsoft suggests submitting mailbox batches about two weeks before the cutover date. The elapsed time is set by discovery and licensing, not by the data transfer.
What Happens to Email During a Tenant Migration?
Mail keeps flowing. The old tenant forwards to the new one while the move completes, in much the same way an Exchange hybrid setup works. The visible change is on day one, when each user has to rebuild their Outlook profile with the new address and let the mailbox resync.
Does Teams Chat History Move Between Tenants?
No. Teams chat folder content does not migrate cross-tenant. A source tenant administrator can still search and export it afterwards using a content search, so it is not lost, but users will not find it in the new tenant. Meetings move but their joining links stop working.
Do I Need to Buy Anything for a Cross Tenant Migration?
Yes. Microsoft requires a Cross Tenant User Data Migration licence for each user you move. It is a one-time per-user fee covering both mailbox and OneDrive, assignable to either the source or target account. Transputec prices these into a tenant to tenant migration at the start, because migrations fail without them.
Can We Keep the Same Email Domain After Merging Tenants?
Not on both. A domain can only be owned by one tenant, so the source and target must use different domain names during the migration. Moving a domain across is a separate step that happens after the users have moved, and it needs its own plan.
This article was drafted with AI assistance and reviewed by the Transputec team. Featured image: AI-generated.
