It is about time, Outlook Mobile on iOS and Android is now supporting S/MIME (Secure Multipurpose Internal Mail Extensions). This means you will be able to send and receive (and as such read) signed and/or encrypted emails with Outlook Mobile.
To start using it, you must have a valid S/MIME certificate off course – manual email delivery of S/MIME certificate is in the roadmap for a near future and automatic deployment using Intune (or any MDM solution) some time after.
Then you need to turn on the S/MIME toogle in Outlook Mobile settings.
The deployment is scheduled starting mid-July but you can already get the feature if you enrol to the Outlook Insider program (http://aka.ms/outlookinsiders (iOS)).