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