[UPDATE 23/11/16] Microsoft have announced a new method of doing what I describe in this blog post.  Matt Shadbolt from the Intune Engineering team has a nice blog post that describe how to use this new process, based on Intune MAM policies.  The below information is still useful though if you want to do more specific restrictions (e.g. iOS vs Android native clients).

What is Intune Conditional Access?

Intune Conditional Access is a pretty neat feature that allows administrators to enforce compliance policies to devices prior to allowing them access to sync their mail with Exchange Online.  … [Keep reading] “Enforcing Outlook App in Exchange Online and Intune Conditional Access”