Move mailboxes through batches using an approach to migration that is staged. The phased migration is usually completed in two stages. In the first stage, the user mailboxes and contents of the mailboxes are transferred over time. After the migration, MX records are updated on the target's server. The rest of the data is moved to the target's server in the next step later.
In this kind of migration, administrators must configure the "Outlook Everywhere" option in your local Exchange Server. Additionally, administrators must remove Unified Messaging (UM) before moving towards Exchange Admin Center. Exchange Admin center. After this is verified, administrators can sync accounts between your local Active Directory domain and Office 365 with Azure Active Directory Sync. Or Microsoft Azure Active Directory Sync Services (AAD Sync) to establish your users on-premises in Office 365. It uses a comma-separated value (CSV) document to create the migration batch based on the mailboxes to be transferred. Learn how to migrate Exchange mailboxes into office 365 Administrators manage the staged migration by using the migration dashboard within the Exchange administrator center. Once an Office 365 mailbox is created and mail forwarding is set up for each user's CSV document, Office 365 sends a status email with details of the transferred and unreachable mailboxes. Once the issue is resolved, the administrator migrates all emails or contacts and other calendar entries to Office 365 mailboxes. When a batch of migrations is completed, the administrator checks that all mailboxes have successfully transferred and converted to on-premises mailboxes that are mail-enabled users. Administrators then assign the licenses for Office 365 users and configure the MX record to point to Office 365 so that email is sent directly into Exchange Online mailboxes. The admin can then create the Autodiscover Domain Name System (DNS) record for the Office 365 organization.
Email Migration tools:-