You need Root and Root Explorer
I downloaded the mail.apk from one ROM (my memory is bad i don't remember) and I copied that mail.apk in \system\app and tried to install it from there. It started the installation but aborted cause I suppose the mail app is in use.
The trick is that after that aborted installation, the option to select Html or Text is now available and I could change it to Html...
Now everything is ok.