Can you check to see where the messages are being sent from?
I presume they are not coming from a compromised mail server or a malware infected machine, but rather some spammer is just using your address as the "from" line.
Happens to me a couple times a year. They send their spew and just randomly select one of the addresses to be the "from" address from that run. You start to notice the problem when your mailbox starts to fill up with non delivery reports.
Not much you can do if this is the case, just wait it out and the scumbags will start using someone else's address. The good news is most people will just let the spam drop in their spam folders and not bother with your address.
Now if you suspect your mail server has been compromised or you may have an infected desktop/laptop, more advice is warranted.