Outlook 2003: how to forward messages and not lose the sender/recipient email addresses

This is also not the smoothest answer, but you can open up the email, and click on the Options... button (or if it's not added to your toolbar, it's menu item View->Options...). You then can see the full internet headers, including all the full email addresses. Copy and paste whatever portion of that text you want into your reply.

Not elegant, but works.

I don't know if this is an option in Outlook 2003, but in 2007 you can use Actions -> Forward as Attachment to forward the original email with all header information intact.

It's a little less convenient for the recipient though, as they now have a .msg file to open instead of reading the forwarded email inline with your message.