Exchange 2010-> Exchange Online Cut Over Migration

When we use E-mail Migration in the Exchange Control Panel to migrate all the mailboxes and corresponding mailbox data from Microsoft Exchange to your cloud-based e-mail organization, it is called a cutover migration . Here all on-premises mailboxes are migrated in preparation for moving your entire e-mail organization to the cloud.

When you migrate Exchange mailboxes to the cloud in a cutover Exchange migration:

The migration service provisions new mailboxes in your cloud-based organization. It creates a cloud-based mailbox for each user account in your on-premises Exchange organization. On-premises distribution groups and contacts are also migrated to the cloud. After the new cloud-based mailboxes are created, the migration service migrates e-mail messages, contacts, and calendar items from the Exchange mailboxes to the corresponding cloud-based mailboxes. 

One of the issues that we might face sometimes is while providing Full Access Rights to the Migration Admin during the migration process. We might face errors like, " Cannot Connect to the Source Mailbox".

One of the solutions for this:

Add the migration account to be used for the migration process to the Administrators, Domain Admins, Enterprise Admins, Exchange Org Admins, and Schema Admins group.  This did not appear to work, unless we simply didn’t give it enough time to replicate /sync.

Instead of granting the Full Access rights on the mailboxes using the Exchange Management Console, run the powershell command:

Add-MailboxPermission "user1" -User "migrationadmin" -AccessRights FullAccess -InheritanceType All.

After this, the migration tool should connect to the mailboxes.