This issue is caused by spammers using a faked/forged email address pretending to be you or compromised email password.
Most spammers will not use a real address because they will get their account closed. This is why they will use either faked addresses or real harvested addresses. Spammers use lists of valid email addresses (such as your own) just for the purpose of diverting bounced messages. If a message bounces, spammers don't want it coming back to them.
Information on reporting abuse, go to:
http://www.abuse.net/
or
http://www.spamcop.net
The solution for this issue is to set up a Sender Policy Framework (SPF) and DKIM record in your DNS zone.