Use the same font and size throughout your email. If you want to accent certain words, use bold and italics.
Use a standard “boring” font. “Fun” fonts and colors just looks messy and unprofessional.
Usually there’s no reason to change the default font of your email client (application, program).
In many email clients you can choose between formats to send your email, typically these three:
(Plain) Text - Text without formatting. No bold or italic, no set font. Email is displayed in whatever font, the recipient’s email client is set to. That will usually be a neutral font or a so called monospace font, what most people would view as a typewriter font.
RTF (Rich Text Format) - Allows for common formatting, similar to that of a simple word processor. Bold, italics, fonts, sizes, colors, bullet lists, numbered lists, but no tables.
HTML - The formatting used by web pages. Pretty much everything including tables, images and links.