Someone using my email (domain) to send bulk mail (spam)? I am getting a lot of returned emails.

This is called "collateral spam" and is mostly used these days by spammers. This is normal. There is nothing you can do to stop someone from using your email address. Anyone on the internet can use another person's email address to send email/spam. All it takes is to fake the email address in the email software.

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 wish you can contact the source network at which the email was generated to report abuse, which is unlikely to result in any resolution usually.

To stop receiving such emails what you need to do is setup proper email accounts from the control panel and disable the catch-all default address so that invalid emails from bounces and virus/worms do not end up in your mailbox.

If you need to get this verified that this was indeed not your account being abused but someone else, please open a support ticket by copy/pasting the headers of the original email as well as the headers of the bounced message and the technical support team can advise you.