On Demand Migration offers the domain coexistence services, allowing your employees to send mail from new addresses and access cross-tenant data before the migration is finished.
Important: Email addresses are only rewritten when sending externally, they will not be written for recipients in either your source or target Office 365 tenants.
Important: The service only works with accounts that have pairs in the target tenant.
Note: Address Rewrite Service requires additional licensing.
To see how to Configure Domain Coexistence please see article How to Configure Domain Coexistence (267344)
For Additional information pertaining to Domain Coexistence please see On Demand Migration Technical documentation