When replying to an email with Outlook it may happen that you write the text using a different font than the one you normally use. For example it could be different in type and size (smaller!). The problem is that Outlook uses the font used by the sender in response.
To force the email to always use the same font you have to go to File – Options. In the configuration mask that appears, select the “Mail” block on the left, click on the “Decorative elements and fonts” button and set the desired fonts.