When mail is sent from a mailbox in Microsoft Office 365 Dedicated/ITAR (vNext) to an on-premises mailbox, an external Out of Office (OOF) message is received instead of an internal one.
This behavior is by default and by design. Microsoft is investigating, and this functionality may change in the future.
To enable cross-forest users to receive internal OOF messages, create a new remote domain for the on-premises domain, and make sure that the AllowedOOFType parameter set to InternalLegacy. To do this, run the following cmdlet: