User Query:
We need to migrate hundreds of Exchange Online mailboxes from one Microsoft 365 tenant to another after an organizational merger. How can we transfer emails, attachments, calendars, and contacts in bulk while preserving mailbox data and minimizing migration issues?
Daniel Carter
Microsoft 365 Migration Specialist
Summary: Two Microsoft 365 tenants rarely remain separate forever. Eventually, a merger, a spin-off, or simply a desire to consolidate IT forces someone to perform an Exchange Online (Microsoft 365) to Exchange Online Migration. It’s not the same as an on-premises-to-cloud migration. All the time, it stays in Microsoft infrastructure. Easier said than done when you bring identity conflicts, licensing gaps, and mail flow disruptions into the equation during DNS changes. This guide will cover why companies end up doing tenant-to-tenant migrations, what Microsoft’s built-in tools handle well and where they fall short, and how a professional tool like the Aryson Tenant-to-Tenant Migration changes the equation.
With the migration of the mailbox items, it helps to migrate all items when you perform an Exchange Online mailbox migration from source to target Office 365 tenants. This type of migration will preserve data integrity and folder structure. Only data related to Exchange Online mailbox content would be part of the migration; neither OneDrive nor SharePoint would be included. The following table describes the key mailbox data that can be migrated and items that might need to be handled separately.
Transfers email messages with their associated attachments.
Preserves supported mailbox folders and their organization in the target mailbox.
Transfers personal contacts saved within the Exchange Online mailbox.
Migrates appointments, meetings, and other calendar entries stored in the user's mailbox.
Migrates mailbox-based tasks and their associated information to the target mailbox.
Transfers notes stored in the user's Exchange Online mailbox.
As the size of Microsoft 365 mailboxes increases, the manual method becomes irrelevant, as it cannot handle bulk migration simultaneously. This is where a dedicated migration tool comes into play. You can use professional software such as the Aryson Microsoft 365 Migration tool to migrate multiple mailboxes by using the Graph API while preserving folder structure and data integrity throughout the migration. You can also perform migration using EWS for Exchange Online. The software also offers multiple advanced filtering options such as date range, duplicate handling, incremental migration, and custom folder naming to make the migration more efficient. You can also use the demo version of the tool before making a full purchase to understand the functionalities.
For a smooth Exchange Online to Exchange Online mailbox migration, having a clear step-by-step reference can simplify configuration, mailbox mapping, and data transfer. It helps administrators understand the migration workflow and avoid common setup errors. Refer to the Aryson Microsoft 365 Migration Tool User Manual for detailed instructions with screenshots.
There are more professional Microsoft 365 migration tools, but they differ in features, pricing, and other factors. Below, we will look at a list of tools such as BitTitan, ShareGate, and Cloudiway and compare them with Aryson to see why Aryson is the most preferred tool in the market.
| Features | Aryson | BitTitan | ShareGate | Cloudiway |
|---|---|---|---|---|
| Folder Hierarchy Preservation | ||||
| Bulk Migration | ||||
| Data Integrity Preservation | ||||
| Duplicate Handling | ||||
| Skip Previously Migrated Items | ||||
| Pricing | $20 per user | $114/10 GB data | $9,995 per year | $29 per user |
A mid-sized insurance company with around 1,400 employees acquired a smaller competitor and was left with two Microsoft 365 tenants and a compliance deadline to consolidate everything into the parent tenant. Their two-man IT team was first trying the native PowerShell way, but mapping over a thousand MailUser objects by hand just wasn’t realistic on that kind of timeline, and early batches kept failing on undocumented permission mismatches. They switched to the Aryson Microsoft 365 Migration Software in week two. Bulk mapping cut setup from days to hours, filters let them skip stale marketing folders that didn't need to migrate, and overnight batch runs meant staff barely noticed a service gap. The full migration finished with about ten days to spare before the audit.
A clean Exchange Online (Microsoft 365) to Exchange Online migration matters more than it looks on paper. Years of mail, calendars, and business communication live in there, and a wrong migration means downtime or data nobody can find later. For a limited number of mailboxes, the native PowerShell approach functions well. After a few hundred, throttling slows everything down, and manual mapping becomes a full-time job. The Aryson Office 365 Migration Tool is justified in this regard. Every mailbox's folder hierarchy and data integrity are maintained, date range filtering is supported for selective migration, duplicate emails are managed, and new messages are picked up through incremental migration all the way up to cutover.
We have close to 1,500 mailboxes and a tight compliance deadline. How can we realistically handle that volume without dragging on for weeks?
Ans. You can use the Aryson Microsoft 365 Migration tool, which runs in concurrent batches rather than one mailbox at a time, so large mailbox counts don't turn into a multi-week task.
Our admin team isn't comfortable writing PowerShell scripts. Is there a way to migrate without scripting everything ourselves?
Ans. That's the main reason teams pick a professional tool such as the Aryson Microsoft 365 Migration tool over the native route. Credentials, mapping, and batch configuration all happen through the interface; no scripting required.
We're worried folders will get flattened during the move and emails will end up dumped into one giant inbox.
Ans. The tool maintains folder hierarchy and carries over as-is. Whatever structure exists in the source mailbox shows up the same way on the destination side.
How do we delete duplicate emails from the target mailboxes after migration finishes?
Ans. The duplicate detection filter is available in the tool; you can enable it during the migration to detect and remove repeated items in the destination mailbox.
We only care about the last two years of mail, not a decade of history. Can we limit what actually moves?
Ans. Date range filtering handles exactly that. Old mail outside the range you set just doesn't get pulled across.
A few mailboxes have personal or irrelevant folders we'd rather not migrate at all. Can specific folders be excluded?
Ans. Yes, filtering works at the folder level too, not just by date, so unwanted folders can be left out of the run entirely.
Users keep sending and receiving mail while migration is still in progress. What happens to messages that arrive after the first sync?
Ans. Incremental migration picks up new items after the initial pass, so nothing sent during the migration window gets missed at cutover, and only new mail is migrated.
Once migration finishes, how do we actually confirm nothing was lost or corrupted along the way?
Ans. A summary report compares item counts against the source mailbox, which makes it easy to catch a shortfall before users are back in their inboxes.
Before committing to a full migration, is there a way to see what's actually going to move?
Ans. Yes, mailbox contents can be previewed ahead of the run, which is worth doing before kicking off anything at scale.
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