Important: S/MIME in Outlook for iOS is now available. The below article has been replaced by product content found at http://aka.ms/omsmime
Secure/Multipurpose Internet Mail Extension (S/MIME)...
Updated Dec 19, 2023
Version 13.0
Blog Post
Quote:
"For automated certificate delivery, Outlook for iOS will only support Intune for enrollment. iOS has two keychains – system and publisher. Any MDM can push certs to the system keychain. However, only first party Apple apps can use that keychain. Outlook only has access to Microsoft publisher keychain. Intune is building a cert delivery channel (outside of the MDM channel), to securely deliver certs into the Microsoft publisher keychain. Third-party MDMs won’t have access to the Microsoft publisher keychain because they are not a Microsoft signed application and thus, can’t deliver certs there for Outlook to use."
Apparently you MUST use Intune, or it wont work for automated deployment.
You can still send the certificate and key as a PFX/P12 attachment by email , and having the user open it in Outlook to install it, after which it should work