All e-mails written with Microsoft Outlook have Calibri as the default font with 11 point font size. If you want to write a message using a different font, you can choose the font you want in the usual way, but you also have the option to change the default font itself.
Click on the File menu, select Options at the bottom, and in the left-hand column, select the Mail category. Under Write messages, click on Letterhead and fonts and open the Custom letterhead tab.
Here you can use the three Font buttons to select which font should be used for new emails, reply and forwarded emails, and plain text messages.