User Configuration > Administrative Templates > Microsoft Outlook 2016 > Outlook Options > Mail Format > Stationery and Fonts
User
HKCU\software\policies\microsoft\office\16.0\common\mailsettings
This policy setting allows you to choose a Stationery font option. If you enable this policy setting, a Stationery font option from the dropdown list will be enforced. If you disable or do not configure this policy setting, the default setting (use theme's font) will be used.