Are you still in the old email format or the new one? To tell, look at the URL in the address bar. If it says Zimbra you are in the old one, if it says Appsuite you are in the new one. If you still have the old one, from the Inbox page go to Preferences>>>>>>Email>>>>>>Compose. there you will see the option to change the default font. If you are in the new format, on the Inbox page click on the gear icon in the upper right corner of the page, then on Settings. Under Mail Settings scroll about halfway down the page and you will see the section for the font.