While the sender's IP is attached to every message, it's also easy to fake or spoof, so you can't really rely on it. Also, most ISPs use a dynamic IP system, which makes it even more difficult. If the messages are sent on different days, they could have completely different IPs.

The long and short of it all, is that email wasn't designed to have these sorts of abilities, and so it doesn't. Any information you might find can't be assumed to be reliable. If you're getting harassing emails from someone, forward them to your ISP. Virtually all of them have some sort of abuse account, which is monitored by people who have the resources to trace these things.