Use a stationary would be my first idea, but that is not quite permanent.
You need to change the font of the Body field in the Memo form of the design.
You can do that only with the Notes Designer client.
Once you have done that, your mail will no longer be completely compatible with the mail standard. Any update that will touch your Memo form will not affect that form any more, which might create stability and/or usability issues.
I've got nothing to hide, and I'd very much like to keep that away from prying eyes.