The Office 365 Message Encryption (OME) provides protection to the sensitive email that was sent or received within the migration user's mailbox. The migration engine via the migration administrator account using Application Impersonation is not able to read the protected emails.
The migrated email will have the subject and the body of the email will be a link to the Office 365 portal for the user to access the email.
More information is available on the Microsoft website.
https://docs.microsoft.com/en-us/microsoft-365/compliance/ome?view=o365-worldwide
