What is a Staged Migration?
A staged migration is an Office 365 migration method used by organizations running an on-premises Microsoft Exchange Server that need to move user mailboxes to the cloud in phases rather than all at once.
Instead of migrating every mailbox during a single cutover, mailboxes are transferred in batches over time, allowing IT teams to manage risk, maintain continuity, and support users gradually. This approach is commonly used when organizations have a large number of user mailboxes or want to minimize disruption during an email migration.
How Does Staged Migration Work in Office 365?
During a staged migration, both the on-premise environment and Office 365 Exchange coexist temporarily, with mail routing configured so users can continue sending and receiving email throughout the migration process.
Directory Synchronization and User Preparation
A staged migration begins by synchronizing user accounts from the on-premise environment to Microsoft 365. This is typically done using Azure AD Connect to enable directory synchronization between Active Directory and the cloud.
At this stage, user accounts are created in Office 365, but mailboxes remain on the Exchange Server. This step is critical because it establishes identity continuity and allows each cloud mailbox to be matched to its on-premise counterpart during the migration process.
Creating Migration Batches
Once identities are synchronized, administrators determine which users will be migrated first. Mailboxes are grouped into a migration batch using a CSV file that lists the selected user mailboxes.
These batches are created and managed through the Exchange Admin Center, allowing IT teams to control the pace of the Exchange migration and reduce risk. This phased approach is what differentiates staged migration from cutover migration, which moves all mailboxes at once.
Initial Mailbox Data Migration
After a migration batch is started, Exchange Online begins the initial mailbox migration. During this phase, email messages, calendar items, and contacts are copied from the on-premise Exchange environment to the Office 365 mailbox. While data is being transferred, users continue working in their existing mailboxes, and normal email communication remains uninterrupted.
Exchange Online also validates key requirements during this phase, such as user account availability and directory synchronization status, to minimize migration errors before proceeding further.
Mail Routing and Forwarding Configuration
As part of the staged migration flow, mail routing is temporarily adjusted to ensure continuity in Office 365. Incoming mail sent to on-premise mailboxes is forwarded to the corresponding Exchange Online mailbox.
This allows users in a migration batch to begin accessing their cloud mailbox while ensuring no data loss occurs during the transition. At this stage, Outlook clients may still reference the on-premise mailbox until the final switch is completed.
Completing Each Migration Batch
Once mailbox data has fully synchronized and administrators verify the migration results, the on-premise mailboxes for users in the batch are converted to mail-enabled users. This step allows Outlook and other email clients to connect directly to Exchange Online.
Users can now access their Office 365 Exchange mailbox without needing to reconnect or reconfigure their email account. This process is repeated for each migration batch until all user mailboxes have been moved to the cloud.
Final Cutover and Post-Migration Tasks
After all batches are complete, administrators finalize the Office 365 migration by updating the MX record so new email is delivered directly to Exchange Online. Additional post-migration steps may include assigning licenses, validating shared mailbox access, and confirming mail flow. In some environments, IT teams may also begin decommissioning the legacy Exchange Server once the migration method is fully completed and validated.
