If you are still in the old email format, from the Inbox page go to Preferences>>>>>>>>Email>>>>>Composing. There you will see the option to make changes to the default font for outgoing messages. If you are in the new format, on the Inbox page click on the gear icon in the upper right corner, then on Settings. Under Mail Settings scroll down and you will see the same thing. But be advised that right now some people, including me, cannot get a change to "take". It's a known issue and they are working on it.