Spammers use software to collect email addresses from the internet. They also use software to guess at email addresses, such as
[email protected],
[email protected],
[email protected], etc).
They then send spam to all of those addresses. However, in order to make their spam messages look as if they've come from a real person, they also use their list of email addresses to complete the 'return address' fields in their emails.
A spammer has entered your address into the 'return address' field of some of the mail he's been sending out. Since many of the addresses he will have sent mail to won't actually exist (because they were only software-generated 'guesses'), you'll get all of the 'bounce' messages from those mail attempts..
Chris