Change the Default Font in Outlook.
1. Select the File tab.
2. Choose Options.
3. Under Mail, Compose messages, choose Stationery and Fonts.
4. On the Personal Stationery tab, under New mail messages or Replying or forwarding messages, choose Font.
5. In the Font box, choose the font, style, size, and color that you want to use. You can see a preview of your changes as you make them.
6. Choose OK three times to return to Outlook.
Note: Your new settings will take effect immediately.