Change the default font or text color for email messages - Outlook
Change the default font, color, style, and size for messages
1. On the File tab, choose Options > Mail
2. Under Compose messages, choose Stationery and Fonts.
3. On the Personal Stationery tab, under New mail messages or Replying or forwarding messages, choose Font.
4. 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.
5. Choose OK three times to return to Outlook.