Email Migration

How to Fix Exchange Migration Errors in Microsoft 365

Phi Hung Vo Oct 10, 2026 869 views

Question details

The user encounters MigrationRecipientNotFoundException and TargetUserAlreadyHasPrimaryMailboxException when migrating mailboxes from an OVH Exchange server to Microsoft 365.

How to Fix Exchange Migration Errors in Microsoft 365

Product: Microsoft 365 Exchange Online
Device & OS: not provided
Scenario: Attempting to migrate an enterprise mailbox from a third-party Exchange host to Microsoft 365 without having access to Active Directory PowerShell modules.
Observed behavior: Assigning a Microsoft 365 license prematurely creates a mailbox, resulting in a primary mailbox conflict. Without the license, the migration batch fails to find the recipient, causing a recipient not found error.

Before you start

Ensure you have global or Exchange administrator permissions in the Microsoft 365 admin center and have documented the exact proxy email addresses used in your source environment.

Solution 1Recommended

Configure the Target Recipient Without Assigning a License

Prevent mailbox conflicts by creating a mail user with the correct proxy address instead of a fully licensed mailbox before completing the migration.

The TargetUserAlreadyHasPrimaryMailboxException occurs when an Exchange Online license is applied before the migration batch finishes, which automatically provisions a blank primary mailbox. Conversely, if no recipient object exists with a matching proxy address, you will receive the MigrationRecipientNotFoundException.

Access Active Users

Create the User Account

Click 'Add a user' to create the target account. Crucially, on the product licenses page, choose 'Create user without product license' to prevent early mailbox creation.

Configure Proxy Address

Ensure the user's primary email address (proxy address) in Microsoft 365 exactly matches the email address on your source OVH Exchange server.

Run Migration Batch

Go to the Exchange admin center, navigate to Migration, and start your migration batch. Once the migration completes successfully, return to the Microsoft 365 admin center to assign the appropriate Exchange Online license.

Configure the Target Recipient Without Assigning a License

Important: If you have already assigned a license and encountered the error, you may need to remove the license, permanently delete the cloud mailbox, and recreate the user.

Solution 2

Create a Service Request for Backend Attribute Clearing

If you continue to experience mailbox conflicts after adjusting the recipient setup, contact Microsoft Support to clear hidden mailbox attributes.

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Frequently Asked Questions

Why do I get TargetUserAlreadyHasPrimaryMailboxException during migration?

This happens when the destination user account in Microsoft 365 already possesses an active primary mailbox. Usually, this is triggered by assigning an Exchange Online license to the user before the migration batch has had a chance to run and sync the source data.

How do I fix MigrationRecipientNotFoundException?

You can resolve this by ensuring the recipient object exists in Microsoft 365 and that its proxy address (primary email address) exactly matches the source mailbox address. The migration service uses this address to map data from the source server to the target.

Can I use Get-ADUser to fix this if I am migrating from OVH?

No. When using a Platform-as-a-Service (PaaS) Exchange solution like OVH, you typically lack access to the underlying Active Directory PowerShell module and cannot execute Get-ADUser. You must manage attributes via the Microsoft 365 admin center or contact Microsoft Support for assistance.

Migration & Recovery