just found this on the net
If you receive some kind of delivery failure notice about an email you never sent, I would be willing to bet this is a result of someone else who uses a computer that is infected with some kind of virus. I frequently receive some of these notifications as well every time there is a virus outbreak.
I will give a brief description of what probably is going on. What often happens is there someone who has either received an email from you or has your email address in their address book. They open up some attachment at some point that has a computer virus and the virus then starts churning out infected emails to all the addresses it can find on that computer. But in order for the virus to "hide" it's true location, the virus will not actually use the computer's actual email address. The virus may randomly grab other email addresses on the computer and use those email addresses in the "From" section of the email it sends out. In other words, it "spoofs" someone else's email address.
So some guy named George gets his computer infected with a computer virus and all the infected emails that are sent from his computer look like they come from a dozen different people, but they don't have George's email address anywhere to be seen. That makes the infected computer hidden to some degree and makes it harder to track down the actual machine that is infected.
But the problem is that if that infected email is sent to an email address that is no longer used or the address is wrong, the email server at that location will send a "Failed Delivery" notice back to the sender. Since the infected email is "spoofing" your email address as the sender, that "Failed Delivery" notice is then sent back to you even though you never sent the email in the first place. This is a side effect from these virus outbreaks.
So you end up receiving some kind of email delivery failure message resulting from an email that you never sent. But the kicker is that the warning message you are receiving may still have the virus attached to it. That is why you should never open the attachment to one of these messages unless you are 100% sure it is a legitimate notice sent to you about an email you actually tried to send.
Cheers Goron