One more possible reason. The email address at your domain, was it a generic name like admin, webmaster, info etc etc. If a domain is live the sender may just be sending to a list of common email addresses.
You should also check you don't have a wildcard entry. This would make "anything" at domain be sent to your email address.
Another thing to think about now is the content of the email, did it contain remote content such as imaged. Images are often used for email tracking. If you allow an image to be displayed, the server will record a "hit" for your address. This confirms that the email address is live and may result in more spam being sent.