Internet email never considered security, so using someone else's email address is probably about the simplest misuse of the internet you can undertake. It would probably take you about 5 minutes searching and reading to learn how to do it. It takes nothing more than running a standard tool on Windows, Unix, Mac or whatever, and typing in the address you would like email to appear to be from. There have been attempts to overcome this, but to do so properly pretty much requires every email system in the world to be altered...