Another way is if you manually make the email up in notepad. You can type up your own headers that say its from whomever and all that.
Lets say you make it look like it is from Bill Gates (firstname.lastname@example.org) and you send it to someone. It is easily figured out who actually sent the message by looking at the Internet Headers. They would list all email servers the message passed through including IPs and the orgin server. And you could just punch in the source IP into a program like NeoTrace and it would tell you where physically the message came from.