Setting up an email cutover migration to Office 365 requires careful planning. Before you begin, here are a few things to consider:
You can move your entire email organization to Office 365 over a few days and manage user accounts in Office 365.
A maximum of 2,000 mailboxes can be migrated to Office 365 by using a cutover Exchange migration. However, it is recommended that you only migrate 150 mailboxes.
The primary domain name used for your on-premises Exchange organization must be an accepted as a domain owned by you in your Office 365 organization.
After the migration is complete, each user who has an on-premises Exchange mailbox also will be a new user in Office 365. But you’ll still have to assign licenses to users whose mailboxes are migrated.