a question on email headers

[Al Davis <ad19@freeelectron.net>]

I am curious how reliable the IP address in email headers is.

For example, here's a header:
(changed a little so I don't give away anyones real address)

Received: from foo.bar.edu ([192.168.99.199])
	by my.computer.net with esmtp (Exim 3.35 #1 (Debian))
	id 195LgM-0001Yv-00
	for <me@my.computer.net>; Tue, 15 Apr 2003 02:20:46 -0600
Received: (from davialbe@localhost)
	by foo.bar.edu (8.11.6/8.11.6) id h3F8Lu930444
	for me@my.computer.net; Tue, 15 Apr 2003 02:21:56 -0600

That's all of the "Received" headers on this mail.  I know this 
one is ok (except for the changes I made myself).  I sent it 
myself, from another system.

My question is about that IP address.  That header was generated 
by my computer.  The address agrees with the one in the log 
file (/var/log/exim/mainlog).  The name does, too.

I would like to believe that is the real address it came from.
How reliable is it really?  How easy is it to spoof?