I have an email with attachments in my inbox that is tagged as having been forwarded on 5/5/09; there is no entry in my Sent Items folder for the email having been sent; there is no entry for the email in either my Draft or Deleted Items folders; there is no entry for the email having been sent from the Exchange Server using the Message Tracking system.
How can an email be tagged as having been forwarded without any trace of it having been sent?

It's possible to make an email look like it came from anyone, and in any way - it's all to do with the mail header! This is the bit that the computer reads the email 'Properties' if you will!
All you need to do is strip out the header info and (if you know how) put in whatever info you want, then send.
If you work in a company - go see the guy that runs the mail server and ask him about it if you want to learn how to do it.

Basically - you can write an email, rewrite the header to say that it's from G.W.Bush@TheWhitehouse.gov.us sent 3 years ago, if you wanted and then send it like normal and that's what the recipient will see when they open it.

Not really much use except for the occasional practical joke! But, there are always those out there who will use the force for evil my young padawan; be careful not to let it be you who strays into the Dark-Side!