If you don't know the sender and you have not contacted them it is possibly one of two things,
1) Spam -- just delete it and forget it
2) a Virus or Worm -- do not open it or even view it in preview pane, just delete it and make sure you have a fully updated Anti-Virus program to deal with it.
They get your email address from various places and use it to trick you into thinking that it is a genuine reply to one of your emails, you then open it and you are infected.
The infection then scans your email address book and sends a copy to each one of your contacts saying virtually exactly what you found only this time when it is recieved by you friends it will appear to be from you.
You may also need to turn off System Restore if you are infected to make sure it is not hiding in there, make sure you turn it back on again when the scan is finished.