First, set up your email account on the Apple device (Setup).
Open the TU Dresden CA page on your Apple device. Tap on the red marked areas to install the 3 certificates.
Now open your "certificate file".
Go to the "settings" to install the profile.
Now click on "Install" and confirm this again in the next two screens.
You will be prompted to enter the password of the certificate, after you have done this click "Next".
Now switch to the mail settings (Settings -> Mail -> Accounts -> TU Dresden Account). There click on "Accounts".
Click on "Advanced Settings".
At "S/MIME" you have the possibility to change the default behavior for new mails.