Exchange Online now has Certificate Based Authentication (CBA) in Preview. I have been waiting for this for a while. CBA will be supported with Microsoft mobile Outlook apps and it will be supported with Exchange ActiveSync (EAS). This is a really important release for organizations who more complex security and authentication requirements when accessing Exchange Online data. Typically organizations that use Smart Cards for all their log-in and access applications have required CBA.
For more information, review this - https://blogs.technet.microsoft.com/exchange/2016/07/19/preview-of-certificate-based-authentication-cba-for-exchange-online/