Open the EAC (Exchange Admin Center) in “Migration” section.
Click “Add migration batch” and type a unique name.
Pick “Migrate to Exchange Online” and hit “Next”.
To migrate, skip prerequisites and set an endpoint. Enter server name and attach CSV file. Schedule batch migration and check status. Ask users to check for disruptions. Update MX record and delete migration batch.

For More Info Check Out: https://www.office365tips.org/migrate-dovecot-to-office-365/