Most Windows based email has a way to show the route including server names the messages took getting to you, but since you didn't say what you're using, I can't be specific.
Here is my situation:
A message is sent, using Blackberry, to an e-mail address. This message is forwarded, using Blackberry, to my e-mail address for me to respond to, but it no longer contains the original sender's address.
Can I retrieve the original sender's address, and if so, how do I do that?