I recently set up EMS for a customer and they wanted to ensure all ios native mail apps were blocked and that all client phones must use the Microsoft Outlook app and that devices are enrolled before they can access corporate email.
Azure conditional access policies make this really simple and the following screenshots ill show how we can create this conditional policy.
Browse to the Azure Active Directory admin center / Azure Active Directory/ Conditional Policies
Firstly Create the Policy
Next we assign what users the policy will be applied to
Select the cloud app – Exchange Online
Select the client app – Active Sync
Select the controls to enforce
Finally save & enable the policy
Now when a client attempts to setup and use the native Apple IOS app , this message will appear in the end users mailbox, the native app will be unusable for sending and receiving messages. The user can then proceed with the device enrollment process.
Microsoft Information Protection 