If you receive message failure emails for emails you didn't send then chances are you don't need to worry.
Viruses and spammers commonly use email addresses from legitimate web sites when sending out malicious emails.
(This is typically called "backscatter" and the following Wikipedia article describes it in depth: http://en.wikipedia.org/wiki/Outscatter)
Unfortunately, anyone can do this as there is no restrictions in place that prevents a computer user from changing the From address in an email program to anything they like.
The good news is that in most cases these emails are not being sent from your computer or through our servers.
The best way to limit the amount of "failure emails" you receive is to turn of the "Catch-All" for the domain.