I am getting hundreds of e-mails everyday from Mail Delivery System, all returned e-mails that I have never sent. Why?
Most probably your system is infected with virus. Latest viruses send e-mails without your knowledge forging Sender and Recipient and bounces go to the Sender/Reply To mentioned in the email. You must scan your system with latest anti-virus software and upgrade your email client to the latest version. You should read the return mails to find out the reason.