Sending a piece of formatted e-mail is an illusion at best. How your message to me reads, on my screen, is ultimately determined by my program, so even if you indicated you wanted a particular font used (which is basically what you're asking for), my program could change it and not show what you intended.
That said... if you use rich text formatting, you have the option of choosing 11 different fonts, one of which is Courier New, which you can see (if your program didn't change it) is a fixed width font, commonly used for formatted code.
Zack