This is called "collateral spam" This is someone faking your email address in a mail application and sending the spam. What matters is the source IP address of the junk mail, which you can check by looking at the headers of the original email, which should be part of the body of the email that was returned to you. If you would like, you can contact the source network at which the email was generated to report abuse. To stop receiving such emails what you need to do is disable the catch-all default address so that invalid emails from bounces and virus/worms do not end up in your mailbox. For any other issues or questions please contact support.
