Action: Deploy Exchange Server to manage Exchange Online users from on-premises - You may choose to enable directory synchronization ...

Action: Deploy Exchange Server to manage Exchange Online users from on-premises - You may choose to enable directory synchronization for your Office 365 organization in order to manage user accounts from your on-premises environment. If you do enable directory synchronization and have Exchange Online users, you need to deploy an on-premises Exchange Server (if you haven't already). Exchange Server is required to manage mail-enabled users in Exchange Online. This scenario is similar to setting up an Exchange hybrid environment. Office 365 provides an Exchange Server license at no cost so that you can use Exchange Server to manage Exchange Online users (see http://aka.ms/hybridkey).