S/MIME with the Outlook app for Apple iOS and Android only possible with devices managed via Intune
If you want to make S/MIME certificates available to your users on the smartphone as well, you may be surprised to discover that this is not possible with the Outlook app unless you also use Microsoft Intune as a management solution for the devices.
Microsoft has now clarified in the article "Sensitivity labeling and protection in Outlook for iOS and Android " that this is due to the respective system architecture.