Block Access from private Devices to Microsoft Apps.

RauschNauti Hello! As mentioned I usually don't configure these settings, but see the tutorial and the other link for step-by-step guidance.

"Learn about using app protection policies with Conditional Access to protect Exchange Online, even when devices aren't enrolled in a device management solution like Intune."

https://docs.microsoft.com/en-us/mem/intune/protect/tutorial-protect-email-on-unmanaged-devices

'Block all email apps except Outlook for iOS and Android using conditional access'

https://docs.microsoft.com/en-us/exchange/clients-and-mobile-in-exchange-online/outlook-for-ios-and-android/secure-outlook-for-ios-and-android#block-all-email-apps-except-outlook-for-ios-and-android-using-conditional-access 

There are a couple of different approaches as you will see.