Staged Exchange migration


Applies to: Exchange Online


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You can use the Migration dashboard in the Exchange Administration Center (EAC) or the Exchange Management Shell to migrate subsets of on-premises Exchange mailboxes and mailbox data to Exchange Online. This type of migration is called a staged Exchange migration because you migrate mailboxes in batches. You use a staged migration if you plan to eventually migrate all your organization's mailboxes to Exchange Online. Using a staged migration, you’d migrate batches of on-premises mailboxes to Exchange Online over the course of a few weeks or months. Your goal would be to permanently move your email organization to Office 365.

Here are a few restrictions and requirements for using a staged migration:

  • You can’t use a staged migration to migrate Exchange 2010 or Exchange 2013 mailboxes to Exchange Online. If you have fewer than 2,000 Exchange 2010 or Exchange 2013 mailboxes in your organization, you can use a cutover Exchange migration. To migrate more than 2,000 Exchange 2010 or Exchange 2013 mailboxes, you have to implement an Exchange hybrid deployment. For more information, see:

  • You have to replicate user accounts from your on-premises Active Directory to your Office 365 organization. To do this, you have to install and configure the Microsoft Online Services Directory Synchronization tool before you can run a staged migration. After mailboxes are migrated to Exchange Online, user accounts are still managed in your on-premises organization and the Directory Synchronization tool synchronizes your on-premises Active Directory with your Office 365 organization. For more information, see Active Directory synchronization: Roadmap.

  • Your on-premises Exchange organization must be an accepted domain of your Office 365 organization.

  • You can migrate user mailboxes and resource mailboxes only. Other recipient types, such as distribution groups, contacts, and mail-enabled users are migrated to Office 365 through the process of directory synchronization.

  • Out of Office messages aren't migrated with user mailboxes. If a user turns on the Out of Office feature before the migration, the feature will remain enabled on the migrated mailbox but the Out of Office message will be blank. People who send messages to the mailbox won't receive an Out of Office notification. To allow Out of Office notifications to be sent, the user will need to recreate their Out of Office message after their mailbox has been migrated.

To learn about and compare other options for migrating mailboxes to Office 365, see Ways to migrate multiple email accounts to Office 365. Or, use the Cloud Only option in the Exchange 2013 Deployment Assistant to help select the right migration option.

For additional options for migrating email to Office 365, see Migrate email to Office 365.

How the staged migration process works

Here's a description of a staged migration. It includes steps performed by an administrator and the migration process performed by Exchange Online. For preparation tasks and step-by-step instructions, see Migrate mailboxes to Exchange Online with a staged migration.

  1. The administrator creates a CSV file that contains a row for each user whose on-premises mailbox will be migrated in the migration batch.

  2. The administrator creates and starts a staged migration batch using the Migration dashboard in the EAC or using the Exchange Management Shell.

  3. After the administrator starts the migration batch, Exchange Online does the following:

    • Verifies that directory synchronization is enabled.

    • Checks that a mail-enabled user exists in the Office 365 organization for each user listed in the CSV file. Mail-enabled users are created in Office 365 as a result of the directory synchronization process.

    • Converts the Office 365 mail-enabled user to an Exchange Online mailbox for each user in the migration batch.

      This part of the process is called initial synchronization. Exchange Online processes up to N migration requests at one time, where N is the maximum number of concurrent migrations that the administrator specified when creating the migration endpoint used for the migration batch. By default, initial synchronization is performed on 20 mailboxes at a time until all mailboxes in the migration batch have been migrated.

    • Configures mail forwarding by configuring the TargetAddress property on the on-premises mailbox with the email address of the Exchange Online mailbox. This means that mail sent to the on-premises mailbox is forwarded to the corresponding Exchange Online mailbox.


      In Exchange 2010 and Exchange 2013, the TargetAddress property can't be modified. This is the reason that staged Exchange migration doesn't support migrating Exchange 2010 and Exchange 2013 mailboxes to Exchange Online.

  4. After it creates the Exchange Online mailbox and configures mail forwarding for each user in the CSV file, Exchange Online sends a status email message to the administrator. This message lists the number of mailboxes that were successfully migrated and how many couldn’t be migrated. The message also includes links to migration statistics and error reports that contain more detailed information. At this point, users can start using their Exchange Online mailboxes.

  5. As part of initial synchronization, Exchange Online then migrates email messages, contacts, and calendar items from the on-premises mailboxes to Exchange Online mailboxes. Exchange Online sends a final migration report when the data migration is complete.

  6. After a migration batch is complete and the administrator verifies that all mailboxes in the batch are successfully migrated, the administrator can convert the on-premises mailboxes in the migration batch to mail-enabled users.

    If a user opens their mailbox with Microsoft Outlook, the Autodiscover service tries to connect to the on-premises mailbox. After you convert on-premises mailboxes to mail-enabled users, the Autodiscover service uses the mail-enabled user to connect Outlook to the Exchange Online mailbox after the user creates a new Outlook profile.

  7. The administrator creates and starts additional migration batches, submitting a CSV file for each one.

  8. After the administrator resolves any migration issues, all on-premises mailboxes in a batch have been successfully migrated, and users are using their Exchange Online mailboxes, the administrator deletes the migration batch.

  9. The administrator performs post-configuration tasks to complete the transition to Exchange Online and Office 365, such as:

    • Assign licenses to Office 365 users.

    • Configure the MX record to point to your Office 365 organization so that email is delivered directly to Exchange Online mailboxes.

    • Create an Autodiscover DNS record for your Office 365 organization.

    • Decommission on-premises Exchange servers (optional).

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