This problem is caused by spammers who are using a forged email address pretending to be yours. Most spammers will not use a real return address because they will get their account closed quickly.
That’s why they will often use either faked addresses or real harvested addresses (almost every large domain will have a ‘email@example.com’, very few will have ‘firstname.lastname@example.org’). They also use lists of valid email addresses just for diverting bounced messages. If a message bounces, spammers don’t want it coming back to their mailbox.
The first thing that you need to do is identify the spammer’s mail host and raise a complaint. More information on the matter is available here.