Cutover 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 provision new Exchange Online mailboxes and then migrate mailbox data to Exchange Online from your on-premises Exchange server. This type of migration is called a cutover Exchange migration because all on-premises mailboxes are migrated to Exchange Online in a single migration batch. Using a cutover migration, you migrate all on-premises mailboxes to Exchange Online over a few days. You would use this migration type if you plan to move your entire email organization to Office 365 and manage user accounts in Office 365. You can migrate a maximum of 2,000 mailboxes from your on-premises Exchange organization to Exchange Online using a cutover migration. This migration method only moves mailboxes, mail users, mail contacts, and mail-enabled groups.

To learn about and compare other options to migrate mailboxes to Exchange Online, see Mailbox migration to Exchange Online. Or use the Cloud Only option in the Exchange 2013 Deployment Assistant to help you select the right migration option.

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

How the cutover migration process works

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


Before you can successfully migrate mailboxes using a cutover migration, your on-premises Exchange organization must be an accepted domain of your Office 365 organization.

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

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

    • It uses the address book on your on-premises Exchange server to identify the mailboxes, distribution groups, and contacts that will be migrated to your Office 365 organization.


      The WindowsEmailAddress attribute is used as the primary key for the cutover migration and changing the WindowsEmailAddress attribute on the on-premises side during a cutover migration isn’t recommended. If the WindowsEmailAddress attribute needs to be changed, we recommend that you remove the target MigrationUser attribute, remove the target mailbox, group and contact, and then restart the migration batch.

    • It provisions new Exchange Online mailboxes. This part of the process is called the 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. By default, you can migrate up to 20 mailboxes at the same time until all the mailboxes in the on-premises organization have been migrated.

    • It creates distribution groups and contacts. The group membership from the on-premises groups is re-created for the corresponding cloud-based groups. All groups will be provisioned as regular (non-security) distribution groups. If an on-premises mailbox fails to be migrated, that user can't be added to a cloud-based distribution group. When that mailbox is successfully migrated, the user will be added to the group.

    • As part of initial synchronization, Exchange Online migrates mailbox data, which includes email messages, contacts, and calendar items, from each on-premises mailbox to the corresponding Exchange Online mailbox.

  3. When the initial synchronization has finished for a migration batch, 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. The migration statistics report also contains an autogenerated, eight-character alphanumeric password for each mailbox that was migrated. Users are required to reset this password the first time they sign in to their Exchange Online mailbox.

  4. After the initial synchronization process is complete, the mailboxes in the on-premises Exchange organization and the corresponding Exchange Online mailboxes are synchronized every 24 hours. This is called incremental synchronization. This means that any new messages sent to the mailboxes on the on-premises Exchange server are copied to the corresponding Exchange Online mailboxes. By default, incremental synchronization is performed on 10 mailboxes at a time until all mailboxes are synchronized. Incremental synchronization continues until the administrator stops or deletes the migration batch.


    During incremental synchronization, deletions made in an on-premises mailbox are synchronized to the corresponding Exchange Online mailbox. This means that if a user deletes an item from an on-premises mailbox that was previously migrated to Exchange Online, that item will be deleted from the Exchange Online mailbox.

  5. After the administrator resolves any migration issues and all on-premises mailboxes have been successfully migrated, and users are using their Exchange Online mailboxes (and are no longer using their on-premises mailboxes), the administrator updates the organization's MX record to point to their Office 365 email organization. This causes email messages to be delivered directly to users' Exchange Online mailboxes.

  6. After the change to the MX record has been propagated and mail is flowing directly to Exchange Online mailboxes, the administrator deletes the cutover migration batch. After the migration batch is deleted, mailboxes on the on-premises Exchange organization are no longer synchronized with Exchange Online mailboxes.

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

    • Assign licenses to Office 365 users.

    • Create an Autodiscover DNS record for your Office 365 organization. The Autodiscover DNS CNAME record has to point to Office 365 in both internal and external DNS after the migration so that the Outlook client will to connect to the correct mailbox.


      In Exchange 2007, Exchange 2010, and Exchange 2013 you should also set Set-ClientAccessServer AutodiscoverInternalConnectionURI to Null.

    • Decommission on-premises Exchange servers (optional).

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