There is no such policy. Outlooks font settings are located in several registry values in the HKEY_CURRENT_USER\Software\Microsoft\Office\11.0\Common\MailSettings key. You can, however, export that key from a machine that already has the format set as you want it to be and then import that .reg file into the Custom Maintenance Wizard. Alternatively, use the Office Profile Wizard to capture and deploy the desired setting.