Mailbox migration between Microsoft 365 tenants: What moves, what doesn't, and how to run it

Mailbox migration takes careful planning, the right tools, and precise execution. If you don’t get it right, you’ll soon be untangling a flood of user support tickets.

This guide covers one scenario: moving Exchange Online mailboxes from one Microsoft 365 tenant to another. You'll see what carries over, what doesn't, how to run the move in waves with delta passes, and how to check every mailbox before you call it done.

What is a tenant-to-tenant mailbox migration?

Mailbox migration means the transfer of email accounts, shared mailboxes, calendar items, contacts, and permissions. 

In this blog, we're going to talk about how mailbox migrations work when the source and destination are both Microsoft 365 tenants. This kind of migration is common after a merger, acquisition, divestiture, or rebrand, when people (and their mail) need to land in a different tenant.

Mailboxes are one layer of a bigger project, and users come first. Migrating Microsoft Entra ID comes first, because every source mailbox maps to a destination mailbox and unmapped mailboxes aren't copied. SharePoint, OneDrive, and Teams content follows.

You can run the mailbox move with Microsoft’s native cross-tenant mailbox migration, which you configure in PowerShell, or with a third-party tool like ShareGate, which maps mailboxes automatically and reports on every one. ShareGate also copies mailboxes within a single tenant, for reorganizations and cleanups.

Not moving between Microsoft 365 tenants? If you’re coming from Google Workspace, ShareGate migrates Gmail to Exchange Online, too. ShareGate doesn't migrate on-prem or hybrid Exchange servers, however, so if that's where you're migrating from, check out Microsoft's native migration tool.

What carries over in a tenant-to-tenant mailbox migration

Most post-migration tickets come down to one question: "Where did my ___ go?"

Tenant-to-tenant migration is one of the more complex mailbox migrations, as you’ll have to deal with mailboxes, company-wide permissions, shared data, and contact information across two or more environments. In other words, things can get hectic fast. And while Microsoft provides some native cross-tenant mailbox migration capability and support for these scenarios, many organizations find that using third-party tools like ShareGate to simplify planning, automation, and reporting makes the migration process simpler and more efficient.

Third-party email migration services cut out the manual work. ShareGate maps source mailboxes to destination mailboxes automatically, lets you pick what to copy, and reports on every mailbox that makes the trip.

Before you plan batches and cutover dates, get clear on what a copy actually moves:

TABLE

Good to know before you start: A few things are handled outside the copy. Public folders and folder permissions aren’t migrated, so plan to re-grant shared mailbox access at the destination. Encrypted emails copy but can’t be read at the destination. Destination room and equipment mailboxes need to exist before you migrate them. And only Litigation Hold settings move, not the data the hold retains. For every edge case, see the Copy mailboxes limitations article.

Archives come along in the same pass

With some migration tools, archives need a separate license or a separate migration task. In ShareGate, they don't. Turn on Copy In-Place Archive and each user's archive is copied alongside their primary mailbox, in the same migration flow, at no extra cost. Archive inclusion is explicit, not assumed, so you decide which copies include it.

A few things to know. Archive and Group mailbox copies use Exchange Web Services (EWS), so make sure ShareGate is on your EWS allow list in both tenants. Once an archive is enabled at the destination, it can take some time before it's visible and usable in Outlook. And if users move emails by hand after a migration, those emails are duplicated during the next incremental copy.

Pre-migration planning and preparation

Taking stock of everything you’ll be migrating before you get started will minimize potential errors and keep downtime as low as possible. 

Here’s what to include in your Microsoft 365 migration prep: 

• Mailbox inventory: Count mailboxes by type and size, and note which have archives. Copy one average-sized mailbox first to see how many items it yields, then size your batches so no single copy goes over 800,000 items.
• Identities first: Migrate your Entra ID users before you copy mail. If you run that identity migration in ShareGate between the same two tenants, your mailbox mappings will be pre-filled.‍
• Permissions and compliance: A global admin consents to ShareGate's Microsoft 365 permission scope once per tenant; after that, an Exchange admin can run the migration. Check which mailboxes are on hold. ShareGate can copy Litigation Hold settings, but not the data the hold retains, and it warns you about on-hold destination mailboxes before you start.‍
• Technical readiness check: Email systems can be surprisingly complicated under the hood. Add ShareGate Migrate to the Exchange Web Services (EWS) allow list in both tenants. Microsoft started blocking EWS by default on October 1, 2026, and Copy mailboxes uses it. Mailboxes on Kiosk, F1, or F3 licenses need a license that includes EWS access before you migrate them. Then plan your MX, DNS, and Autodiscover cutover, which is a Microsoft 365 admin step outside ShareGate.‍
• Decommissioning and data retention: Decide how long you’ll keep your old environment live, and create a data deletion framework. Once everything is safe and verified in an Exchange mailbox, you can begin decommissioning your legacy platform.‍
• Let your end users know: Let users know what you’re doing. Offer rough timelines, support lines to help them through the transition, and training sessions to bring them up to speed on their new mailbox environment. If you create new Teams meeting links at the destination, warn people that they'll get a second invitation for the same meeting during the migration.

Run your Exchange Online migration in waves

With your prep done, the migration itself comes down to two kinds of passes: one full copy, then as many incremental (delta) copies as you need to stay current until cutover. Users keep working in their old mailboxes the whole time, so each delta pass picks up only what changed since the last one.

Every pass is also a checkpoint. Each one produces a migration report, and reading it before the next pass is how small problems stay small. Want the click-by-click version in ShareGate? See our step-by-step guide to tenant-to-tenant mailbox migration.

The 4 steps of a mailbox migration

Each step below ends with a look at the migration report, so issues surface while they affect a handful of mailboxes, not hundreds.

1. Test one mailbox: Copy one average-sized mailbox and check its report to see how many items it yields and whether anything errors. Use that number to size your batches so no single copy goes over 800,000 items.
2. Run the initial copy in batches. Map source mailboxes to destination mailboxes, choose what to copy (emails, calendars, contacts, rules, In-Place Archives, Litigation Hold settings), and start. If you’re moving users in waves, turn on Coexistence in the same copy options so mail sent to migrated users’ old addresses reaches their new mailboxes. The in-app report updates live, so you can start on errors before the batch finishes.
3. Fix, then re-run. Resolve what the report flags, then run an incremental copy. We recommend starting with an incremental migration to resolve issues from your initial copy. Run it as often as you need.
4. ‍Cut over. Run a final delta pass, then switch MX, DNS, and Autodiscover in Microsoft 365.

If a pass stops partway, you don’t start over: incremental copies work even if the previous migration was incomplete or canceled.

Keep delta passes clean

Delta passes work because ShareGate Migrate remembers which items it's already copied for each mailbox. These rules keep that memory intact:

• Run every delta pass from the same ShareGate Migrate instance, with the same mappings, as your first copy. A follow-up copy from a different machine duplicates all items at the destination.
• Leave the destination alone until the migration is done. Changes made there can be overwritten during a delta pass.
• Expect cancelled meetings to need attention. Events removed or cancelled at the source after the first copy aren't removed or cancelled at the destination.

Follow those and you can run as many delta passes as the project needs, they're included in the same annual price. For more on timing passes around cutover, read how Exchange Online delta migration keeps mailboxes current through cutover.

Check the migration report after every pass

ShareGate Migrate's in-app migration report updates live while a copy runs and stays available afterwards from the Tasks screen. The All mailboxes tab lists every mailbox with its status, source, destination, item counts, and start and end times. The Errors and Warnings tabs group issues by type, showing how many items and mailboxes each one affects, with a Learn more link when a related help article exists.

You can export exactly what you need: a single mailbox, a single issue group, everything in the Errors or Warnings tab, or the whole migration. The full downloadable report is a .zip with a summary file plus one CSV per mailbox. Filter the Status column, then read the Details, Errors, and Warnings columns to see what happened to each item.

When the report flags something, fix the cause and run an incremental copy rather than starting over. Then check the next report to confirm it cleared before you move on to the next wave.

Post-migration tasks

During post-migration, you’ll check that everything’s working as it should, verify data integrity, and fully cut away from your legacy environment. Here are the main post-migration tasks: 

1. Inspect licenses: Check to see if users have working M365 licenses and that their mail systems work as expected. Send out some test emails from internal and external accounts to verify. 
2. Validate mailboxes: Work through any remaining errors and warnings in your final migration report, then spot-check contacts, calendar events, and folders. Re-grant folder permissions at the destination, since they aren't preserved during the copy.
3. Provide support: Offering quick-start guides, tutorials about the new email environment, or even just a helping hand can bring people up to speed much quicker than just sending their new account details.
4. Decommission old environments: Once the migration is complete, it’s time to wind down your legacy email system. Archive anything you might need, revoke permissions, and remove access. If you used ShareGate's mail coexistence, it stays active until you decommission the batch, so turn it off as part of this step.

Mailbox migration best practices

As the saying goes: Fail to prepare and prepare to fail. Staying one step ahead can make all the difference between a smooth migration and a support headache.

Here are a few best practices to keep in mind when migrating to Exchange Online:

• Ensure data integrity: Check and double-check each mailbox's migration report before you decommission the source tenant. ShareGate only copies, so your source mailboxes are still there if you need to re-run.
• Minimize downtime: Use phased rollouts and delta passes to minimize disruption and downtime, especially for large-scale migrations.
• Maintain compliance and security: Check that your destination tenant has the DLP, multi-factor authentication, and data retention policies you need. ShareGate copies Litigation Hold settings, but retention policies and eDiscovery holds need to be set up again at the destination.
• Plan for throughput: A single mailbox copies at up to 1.5 GB of data per hour, so your largest mailboxes set the timeline. Copy several mailboxes at once, and spread the work across machines when you need more speed. With PowerShell, ShareGate can copy up to 128 mailboxes at once.

Want more? Read our cross-tenant mailbox migration best practices.

Simplify your mailbox migration with ShareGate

If there’s one thread through this guide, it’s that tenant-to-tenant mailbox migrations reward repetition: copy early, catch up with delta passes, and check the report each time. Get those right and cutover is just one more delta pass, not a leap of faith.

ShareGate is built around that workflow. It maps mailboxes for you, brings In-Place Archives along in the same pass, and lets you run as many delta passes as the project needs, all without PowerShell. It also covers Entra ID, Gmail, SharePoint, OneDrive, and Teams.

Mailbox migration doesn’t have to feel risky. With ShareGate, your source is never touched, re-runs are easy, and every mailbox gets a report. Start a free trial or request a demo today.