Philipp Bieber - Outlook for iOS and Android supports Exchange on-premises already today. This solution synchronizes a portion of the mailbox data in Azure and leverages basic authentication with the Exchange ActiveSync protocol. It does not support EMS capabilities like Azure Active Directory Conditional Access and Intune App Protection policies.
We are building support for hybrid modern authentication with Outlook for iOS and Android against on-premises mailboxes, which brings OAuth, Conditional Access, and Intune App Protection policies. This solution synchronizes a portion of mailbox data into your Exchange Online tenant. See https://blogs.technet.microsoft.com/exchange/2017/09/27/tap-outlook-mobile-support-for-exchange-on-premises-with-microsoft-enterprise-mobility-security/ for more information. We'll be GAing this feature soon.
Microsoft