These instructions apply to Microsoft Office 2010 or 2013 running on Windows 7:
When you compose a new rich-text email message, the formatting is based on the template file named NormalEmail.dotm. In Windows 7, you'll find that file here:
Once you find the file, the first thing you want to do is create a backup copy of it under a different name somewhere then put it somewhere out of the way. Once you do that, open the original in Word (it opens in the form of a default blank .docx file). Make the font changes that you want applied to new emails. Once you are happy with the formatting, "Save as..."
In the Save as dialogue box, click the "Save as type" dropdown menu and change it to .dotm. Save this new template file on your desktop and close out of any MS Office program you've got open. Finally, move the revised "NormalEmail.dotm" file from your desktop to the templates folder and confirm that you want to replace the file.
The formatting changes you made to the template should now be reflected in new rich-text emails. Make sure your emails are being composed in Rich-text format by going to the "FORMAT TEXT" tab (in Office 2010 and 2013) as show in the screenshot below.
You can also change the formatting for plain-text messages by altering the default stationery. This will only affect the format as seen by you, the sender. The format in which the will recipient see it will depend on his/her email program or service.
Changing PT stationery format: File tab -> Options -> Mail -> Stationery and Fonts... -> Personal Stationery tab -> Font button under "Composing and reading plain text messages.