Viruses and spammers will spoof (fake) the "From" line in the email address. The email does not really come from your server and is not being sent from your account. However, if the mail user does not exist on the server the email is being sent to, that mail server will reply back to the "From" email address.
Unfortunately, there is nothing that can be done to stop viruses and spammers from using this tactic. The only traffic you are receiving from this is the traffic generated from the bounced message.
