alwayssign gpo setting

Hi.

We would like to set a default policy for our users that enables e-mail signing. 

I have enabled the "Sign all messages policy" which seems to work.

https://admx.help/?Category=Office2016&Policy=outlk16.Office.Microsoft.Policies.Windows::L_Signallemailmessages

We would however like to allow users to remove the option for individual mails e.g. only setting t he value as default but now restricting users from changing the value.

Any idea how to accomplish that?

Thank you.

Kasper