This is common – as fonzi says, they have sent out spam emails which appear to come from your email domain.
I don’t think there’s anything you can do about it unfortunately, except perhaps set up a rule that removes ‘delivery failure notifications’ to a separate folder so they’re not so annoying.
Also, those messages will normally come back to randomcrap@yourdomain.com, i.e. [email protected]. So what I do is I have specific rules for the [email protected] that really exist (webmaster@, myname@, etc) and anything else is sent to a Suspected Spam folder.
It doesn’t solve the problem, but it makes it a lot easier to clean up.