Adding personal certificate to iPhone mail for your iCloud.com mail account

I was wondering if somebody knows of some way how to use a personal certificate to validate outgoing my.name@icloud.com emails using the standard mail app on my iPhone?

I have searched the web thoroughly, and although it is quite easy to setup on Mail for MacOS, I cannot find a way to do the same on Mail for iPhone. At least, not for your my.name@icloud.com account. It is strangely enough perfectly possible for any gmail.com or Exchange mail accounts, just not for iCloud.com.

Is there any reason why Apple would not support this on iPhone?

Your help is appreciated!

[Edited by Moderator]