Send encrypted icloud.com email with S/MIME on iOS?

I obtained a certificate for my icloud.com email address and imported it into iPhone/iPad/Mac. It's trusted on all devices.

When I send an email @icloud.com from Mac to myself, I could decrypt and read it on iOS. However from iOS I have no way to send out encrypted icloud.com emails. It's just plain messages.

I understand that if we configure an email account using the IMAP configuration, we have a way to switch on the S/MIME in Settings/Mail, Contacts, Calendars on iOS. But my iCloud account was set up under Settings/iCloud. There seems to be no explicit S/MIME setting for the icloud.com email account.

Could anyone help please? Thanks in advance.