It's a virus Jim, but not on your machine!!!
(apologies to the good Captain K)
It is probably a virus infecting the machine of someone who has you in their address book. The virus is then trying to replicate by sending out emails, and picking a random name from the address book to "spoof" as the sender.
So you did not send out the mails, but you get the failure messages.
Sometimes you can track the IP of the sender from the failure notice.