An email itself contains nothing but plain text. Therefore there is no such thing as a font in an email.
You only can decide the content of that text. If you simply send plain text as you did above the reader of the mail can set his terminal or mail reader to a specific font to display that text.
You can also send html code in your mail, which again is noting but text. In that case the reader of your mail has either to manually interpret your html code or use a mail reader which renders your mail like a web page.
It is also possible to add both a plain text and html version to your mail in this case the reader can decide whether it shows the plain text or html part of your mail.
If you want more details maybe RFC 1341 will answer all of them.