I'm attempting to write a tutorial on email forgery and detection. I've got the basic stuff done, but I was wondering if it is possible to spoof your IP address when sending an email, so that the mail server thinks the email is being sent from another computer. The reason I was thinking about this was because when you connect to port 25 of a mail server using SMTP you issue a whole bunch of commands to the server, but while the server's responses are useful, they aren't vital to the process. Theoretically (well to me anyway ) you could craft packets such that the mail server sends the responses to another IP address, while it's actually you who's sending the email. The advantages of this would be that in the 'received' headers your actual IP address will not appear, and also when a mail server is configured to only allow certain IP addresses to send email, you could spoof one of these addresses and send email from that mail server.

Of course, this may not be possible at all due to something I've overlooked. If someone could help me out with this, it would be greatly appreciated.