You can't be positive of the route that the email has travelled, but here's an idea for you. Not sure if it will work, SR knows a ton about the protocol and will give you more valid info.
Say you send an email to [email protected]
, and you want to know the route it takes there. First off, you could just do a regular tracroute or visualroute to see where it goes, specifying the mail server as the destination. But I don't know if that's the actual route it would travel. So try sending an email to [email protected]
, which has a high probability of not exisiting. So it'll get bounced back to you, then you can look at the headers and see what route it took to get to you. Maybe it'll even have the original headers in it with all that data. Not sure if that works, just an idea.