Sadly there's little we can do to stop that. Spammers can easily change the look of emails so that it seems you sent them to yourself. They probably do it with software now, for example an email address generator which will create random email addresses, send the spam, use that same email address as the From field. They don't care if 99% of the emails they send are returned as 'not found'. They are only interested in the ones that get through.
We can help ourselves. Have a look at the do's and don'ts I posted here;
http://forums.cnet.com/5208-6638_102-0.html?messageID=3343207#3343207 and then have a look at Jimmy's Tips in the post here;
They are 'general' tips, but still worth reading.
I assume not much can be done about SPAMERS who use your email address in their "from" field. You can't block it because you will block ALL your "good" email. I have reported it to Yahoo a while back, now it's happening with my GMail address. This is really starting to "TICK" me off!!!