Im in the process of setting up the Exchange Online Protection but have come up with a query.

The install will be a pure standalone environment with all mailboxes being on-premise. I need the setup purely for spam/malware protection.

The question is do I need to add all the on-premise users into the Office 365 portal? The instructions tell me so but surely I can setthe domain to be Internal Relay and they will forward on to the on-premise?

Thanks in advance