It's probably spam sent by some outfit in China or South America (or even the USA) using your address. I've had plenty of such. The problem is, they then get you added to other spammers' lists.
The best fix I've found is to spend $30 a year on a Spamcop account. Mine filters all incoming mail against Spamcop's blacklist (and some other parameters); as a result I get two or three spams a day, and about 500 a day get caught in the Spamcop filter, where I can check them quickly and delete the lot in one go (occasionally a legit one gets caught because that ISP has been used by a spammer).
If I could, I'd ditch the various addresses I use and start again. Trouble is, even new addresses soon get hit. I registered a new domain, and before I used it I was getting spam to info@ accounting@ and mail@ it. You can't win.