How to install a self signed certificate on an iPhone/iPad - impossible to encrypt e-mails - 5 years issue

I never had an answer to the following question since 2020.

Do not redirect me to apple instructions I already tested all without success

I have ios 26, iPad os 26, Mac OS 26 installed, an iPhone 17 and and iPad Air M2

I create my own self-signed certificates (using apple keychain on Mac OS).

With Mac OS I was able to send & read encrypted e-mails with my self-signed certificate.

But wit iOS and iPad OS it is not possible since a few years.

I can read encrypted e-mails sent to me but I cannot send encrypted e-mails.

Of course I had all the certificates installed (mine and those of my correspondants) and they are all trusted (in preferences/ general)

I know that everything is done to prevent users to use self-signed certificates not validated by a distant certificate authority.

Nevertheless, is there any solution to force iOS/iPad OS to recognise your own self certificate (private key+public key) and to encrypt the e-mails sent ?