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An Exhange staged migration moves email mailboxes in batches from one Exchange Server to another on-premise Exchange server or to a cloud-based email service.
The basic staged migration process starts with the destination Exchange Server receiving information about the content to be migrated. For online destinations such as Office 365, the Microsoft Online Services Directory Synchronization tool must be installed and configured beforehand. The Exchange administrator must then create a CSV file containing details of the email accounts to migrate. Once an endpoint is established, the administrator invokes staged migration using the CSV file as a roadmap.
Staged migrations are performed through the migration dashboard in the Exchange Administration Center or using Exchange Management Shell scripts and CSV files to handle batch migrations over time.The staged migration takes several hours to several days to complete, depending on the number of mailboxes and the volume of content that must be migrated.
Staged migrations accommodate up to 2,000 email mailboxes. Staged migrations typically impose little disruption for users, though users may be required to restart Outlook and change their passwords when the migration is finished -- since mail will no longer be received at the old server.
The Exchange administrator is informed when the migration finishes, including the number of mailboxes migrated, the number of migration errors and links to statistics with further information. The administrator may take corrective or optimization actions. They then delete the batch to ensure that email traffic redirects to the new server. This process repeats for each batch of mailboxes. At that point, the old Exchange server can be decommissioned.
Microsoft Exchange 2010 and 2013 cannot use staged migration when more than 2,000 mailboxes are present, and these versions cannot make staged migrations to Exchange Online. Later versions of Exchange email servers must use cutover migration, which is limited to Exchange deployments of 1,000 mailboxes, or use hybrid deployments which support local and online deployments to coexist until administrators can organize migrations.