Spam filtering is getting quite effecient these days. The people sending spam now need to be more creative in the way they get spam delivered to your mailbox.
One of the ways that they do this, is to send email to an address which they know will not work. On the face of it, this seems to be entirely pointless.
The nub here however is that they give your email address as the from addresss. When the email cannot be delivered, it is then sent back to the address given in the "from".
By doing this, the spammer gets the email into your mailbox.
So if you get a message bounce back, and you did not sent it, there are two things to consider:
1. Is your computer infected with a virus?
(Some viruses exist on your computer just to send email, and you may well get hundreds of bounce-backs.)
2. Was the spammer targetting you indirectly?
(You will receive an odd email here and there.)