I am getting bounced emails that I did not send out in the first place. How come?
This is most likely caused by a spammer or a virus that has forged your email address and is sending out emails with the forged email address.
As the email appears to come from a legitimate address, the email may be delivered to unknowing clients. As these emails appear to come from your email address, they are bounced back to the sender when the emails cannot be delivered.
Unfortunately there is not much that can be done other then deleting the emails. More information on forged emails.
If you are concerned about a virus on your machine you can download and run Sophos Anti-Virus.